Leibniz on Fundamental Ontology: Idealism and Pedagogical Exoteric Writing
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Leibniz’s views on fundamental ontology are a matter of great dispute in the secondary literature. Commentators have taken Leibniz to be committed to (1) a monadological metaphysics, (2) an ontology of corporeal substances, (3) different views at different times, and (4) inconsistent views. Monadological interpretations have been criticized for providing implausible readings of texts that seem to indicate a commitment to (2). I argue that a strong defense of (1) can be provided if we pay careful attention to Leibniz’s use of rhetorical strategies. Leibniz thinks it would be a mistake to straightforwardly present (1); he chooses instead to use pedagogical exoteric writing to gradually reveal (1) to his interlocutors. With this framework in place it is possible to provide perfectly natural monadological readings of texts that have been taken to indicate a commitment to (2).
One of the most controversial issues in Leibniz scholarship is his view on substance and fundamental ontology. In the last few decades a divide has emerged between “idealist” and “realist” interpretations of Leibniz’s theory of substance. Idealist interpretations claim that Leibniz endorsed an ontology of simple, immaterial, mind-like substances.Realist interpretations, on the other hand, claim that Leibniz endorsed an ontology of organisms or corporeal substances.These are not the only interpretations of Leibniz’s views on fundamental ontology that have been set forth in recent years, however. Developmental interpretations, according to which Leibniz endorsed versions of both of these theories at different times in his career, have been proposed as alternatives to traditional idealist and realist interpretations.Other commentators have suggested that Leibniz is simply inconsistent on this issue, or that he was ultimately unsure about which theory of substance to endorse.Recent developmental and inconsistency readings have highlighted the fact that Leibniz says a wide range of things about substance throughout his corpus. One might be tempted to conclude, in light of these textual difficulties, that it is no longer tenable to maintain that Leibniz attained a stable view on fundamental ontology.The only way to do that, one might think, is by simply ignoring recalcitrant texts, or by trying to implausibly read one’s interpretive theory into them.
In a provocative recent paper titled "Leibniz's Conciliatory Account of Substance," Jeffrey McDonough has presented a novel interpretation of Leibniz’s ontology that purports to do something that traditional idealist and realist interpretations have failed to do, namely to make good sense of “the whole range” of Leibniz’s texts on substance (2013: 8). McDonough provides a nuanced account purporting to show that Leibniz thought that a purely philosophical analysis of the notion of substance shows that both simple substances and organisms exist and satisfy all of the philosophical requirements for being substances.However, the notion of substance was not a purely philosophical notion in the early modern period. Leibniz was well aware that a variety of theological and political commitments informed people’s views about fundamental ontology. According to McDonough, Leibniz wanted his philosophy to be able to accommodate these extra-philosophical concerns. In keeping with this conciliatorism, as McDonough describes it, Leibniz held that the philosophical analysis of substance reveals two candidates for the true substances, namely immaterial simples and organic unities (2013: 17). Everyone should concede this point. However, Leibniz thought that the extra-philosophical commitments of some of his interlocutors might tip the scales in favor of one or the other of these two candidates as the true substances. Accordingly, texts that suggest organic unities are the true substances should be read as expressing Leibniz’s conviction that a broadly Aristotelian view of substance is philosophically tenable. Texts that suggest that immaterial simples are the true substances should be read as expressing Leibniz’s conviction that a broadly Platonic view of substance is philosophically tenable. Texts suggesting that both organic unities and immaterial simples are substances should be read as expressing Leibniz’s philosophical convictions “in their clearest terms” because they indicate the simultaneous existence of candidates answering to the demands of both conceptions of substance (McDonough 2013: 8).
McDonough’s interpretation is ingenious, but I think it should be resisted. The greatest shortcoming of the interpretation is that it cannot provide plausible readings of particular passages when they are considered in their proper context. I demonstrate this through an analysis of Leibniz’s correspondences with Burcher De Volder and Bartholomew Des Bosses. I show that if Leibniz was committed to the view that McDonough attributes to him, it is utterly mysterious why Leibniz would insist upon an idealist ontology in his later letters to De Volder. Similarly, McDonough’s interpretation cannot make good sense of Leibniz’s remarks on per se unity and the vinculum substantiale in the correspondence with Des Bosses. Given that these are Leibniz’s two most extensive and detailed discussions of substance, an interpretation that cannot make good sense of what Leibniz says in them is not tenable, in my opinion.
McDonough is, nevertheless, quite right on several key points. First, he is right to insist that Leibniz can say that organisms are real and exist, even if they are not substances, strictly speaking. Second, he is right to insist that careful attention to Leibniz’s rhetorical strategies is essential to making sense of Leibniz’s remarks on fundamental ontology. We just need to look elsewhere for an account of what the rhetorical strategies are. A more plausible account can be provided if one pays close attention to what Leibniz says about esoteric and exoteric presentations of his philosophy. A number of insufficiently appreciated texts show that Leibniz thought it would be a mistake to straightforwardly present metaphysical views that are far removed from received opinions because they would be misunderstood and summarily rejected.Nowhere was this danger greater than in the case of an ontology of simple, immaterial, mind-like substances. Leibniz thought that nearly everyone was at least implicitly committed to the view that bodies are substances, and that they were not accustomed to conceiving of “purely intelligible” things (G 6: 502–503; AG: 189). When presented with an ontology of immaterial simples such people would typically misunderstand the view and quickly dismiss it. To combat this problem Leibniz composed exoteric texts that were less than fully explicit about his idealist ontology, often overemphasizing the extent to which bodies are real in his system. Leibniz’s aim was not, however, to permanently hide his real views from the public, as Bertrand Russell infamously suggested.His aim was pedagogical: he used exoteric texts as intellectual stepping-stones that would enable his readers to gradually progress from received opinions to esoteric truth. This nuanced understanding of Leibniz’s use of pedagogical exoteric writing provides a promising template for interpreting Leibniz’s remarks on substance in a wide range of contexts.It also provides a strong response to the criticism that idealist interpretations don’t let the texts speak for themselves, dogmatically reading the theory of monads into texts that are not really in keeping with it.
This paper will proceed as follows. In Section 2 I provide an overview of McDonough's conciliatory account of substance. In Section 3 I provide a critique of McDonough's account. In Section 4 I provide a sketch of Leibniz’s views on pedagogical exoteric writing. In the final section of the paper I develop an interpretive framework for making sense of Leibniz’s remarks on fundamental ontology and defend an idealist reading of the correspondence with De Volder. Let me state up front that it will not be possible to provide a full defense of Leibniz’s commitment to idealism in his later years in this paper. To do that I will need to apply the interpretive framework developed in this paper to a wider range of texts. I leave that as a task for future research.
2. McDonough’s Conciliatory Account
McDonough suggests that we should view Leibniz’s remarks on substance from the perspective of what he calls Leibniz’s “overall conciliatorism.” This conciliatorism consists of two pillars, his “theological irenicism,” and his “philosophical eclecticism” (McDonough 2013: 5–6). Key to the former is the distinction between truths that can be known by unaided human reason and truths that are “above reason.” Truths that are above reason, such as those concerning the Christian mysteries, can only be known through revelation. Although these truths cannot be known through unaided human reason, Leibniz insists that they must be consistent with the truths that can be so known.This theological irenicism allows Leibniz to “insist that there are philosophical truths that everyone must accept while still granting that there may nonetheless be irresolvable differences of opinion that—as long as they are consistent with truths that can be conclusively established—should be tolerated” (McDonough 2013: 5). Leibniz’s irenicism provided a framework for his lifelong efforts to reconcile the various Christian sects.
Leibniz’s philosophical eclecticism, as McDonough understands it, concerns the philosophical truths that can be established conclusively. It involves not merely collecting and citing the views of his predecessors, but showing how a range of seemingly disparate views can be reconciled “by revealing the deeper truths that his predecessors had only partially grasped or confusedly expressed” (McDonough 2013: 6). This philosophical eclecticism was part of what drove Leibniz to attempt to reconcile ancient and modern philosophy, among other things.
Leibniz’s philosophical eclecticism bears directly on his treatment of substance, according to McDonough. He plausibly takes Leibniz to belong to a long lineage of thinkers who held that human beings were paradigmatic examples of substances. There was, however, a long-standing divide over how to properly conceive of human beings. On a largely Platonic conception of substance, you and I are essentially immaterial minds, which might happen to be related to bodies. According to a more Aristotelian conception of substance, you and I are essentially unions (or composites) of substantial forms (or minds) and bodies (McDonough 2013: 2–3). Initially, this divide between Platonic and Aristotelian conceptions of substance might seem to be reproduced rather than resolved in Leibniz’s writings. As realist interpretations have emphasized, some texts suggest that Leibniz is committed to an ontology of corporeal substances, which are conceived of as unions of substantial forms and organic bodies.Idealist interpretations, on the other hand, find a more Platonic conception of substance in texts that seem to indicate a commitment to an ontology of immaterial simples.
But if we pay careful attention to Leibniz’s conciliatorism, McDonough thinks we will be able to see that something subtler is going on. In order to appreciate it we must understand his two-stage metaphysical analysis of the created world. For our purposes, only an overview of McDonough’s detailed analysis will be required. McDonough takes Leibniz to argue, in the first stage, that the created world is exhaustively constituted by organic unities ad infinitum. On his reconstruction, the gross bodies of everyday experience are materially constituted by organic unities, that is, by substantial forms united to organic bodies. These organic unities are themselves constituted by smaller organic unities, which are constituted by smaller organic unities, ad infinitum (McDonough 2013: 9–11).
The second step of Leibniz’s metaphysical analysis is designed to show that immaterial simples constitute the organic unities of the first-stage analysis. He takes Leibniz to hold that immaterial simples must exist in order for organic unities to exist, because the unity in an organic unity must come from a unifying substantial form (or immaterial simple). Additionally, McDonough takes Leibniz to hold that organic per se unities derive their being from their substantial form and their organic bodies (or secondary matter). The organic body of any organism is itself an aggregate of smaller organisms. The being of these organic unities will, in turn, be derived from their respective substantial forms and their secondary matter. If, at each stage of the analysis, the matter of an organic unity “is further analyzed in terms of form and matter, at the end of analysis, we must arrive at immaterial forms alone” (McDonough 2013: 14).
The two-step analysis of the created world shows that organic unities and immaterial simples must exist, and that as per se unities and principles of activity (among other things), both organic unities and immaterial substances satisfy all of the philosophical conditions for being substances. One might think that it should follow from this that organic unities and immaterial simples are both substances, strictly speaking. But McDonough resists this conclusion. He resists it because he does not think that substance was a purely philosophical notion for Leibniz. He takes Leibniz to belong to a long tradition of thinkers for whom philosophical considerations about substance were deeply intertwined with a range of religious doctrines such as the trinity, the Eucharist, and the resurrection of the dead. McDonough’s Leibniz was willing to allow that these extra-philosophical concerns might “tip the scales for some interlocutors in favor of counting one set of candidates as the world’s uniquely true substances at the expense of the other set” (2013: 17).
McDonough’s interpretation is carefully constructed to make sense of Leibniz’s seemingly diverse remarks on substance. He focuses on three sorts of texts: those that appear to affirm an ontology of organic unities, those that appear to affirm an ontology of immaterial simples, and those that appear to affirm an ontology of both organic unities and immaterial simples. Texts of the first sort should be read as focusing on the first-stage of Leibniz’s metaphysical analysis of the created world. Their aim is to reveal candidates for substances for those who are committed to an Aristotelian conception of substance. Texts of the second sort should be read as drawing on the second stage of his metaphysical analysis of the created world, which is supposed to reveal candidates for substances for those who are committed to a Platonic conception of substance. The deeper truth, which Leibniz does not always reveal to his interlocutors, is that both organic unities and immaterial simples exist and satisfy all of the philosophical requirements for being substances. Hence texts of the third sort should be read as expressing Leibniz’s philosophical convictions in their clearest terms. Yet because substance was not a purely philosophical notion, his considered position is not that organic unities and immaterial simples are substances, strictly speaking. This is, for McDonough’s Leibniz, not something that unaided human reason can conclusively establish. One’s extra-philosophical commitments can lead one to reasonably endorse an ontology of organic unities or immaterial simples as the unique true substances (McDonough 2013: 17). This is not problematic so long as the extra-philosophical commitments contain nothing that is contrary to reason.
3. Critique of McDonough
There is no denying the ingenuity of McDonough’s interpretation. He succeeds, I believe, in constructing a creative interpretive framework that could, in principle, allow us to make sense of many of Leibniz’s initially puzzling remarks on substance. It is notable, however, that McDonough’s account is left at the level of a general framework. He tells us how we should interpret different sorts of texts—those that seem to endorse an ontology of simple substances, those that seem to endorse an ontology of corporeal substances, and those that seem to countenance simple and corporeal substances. But he does not apply this framework to particular discussions understood in their proper context. In order to assess the plausibility of the framework we need to descend to the level of particular texts and try to determine whether the interpretive framework can make good sense of them. In this section I will argue that the framework does not make good sense of Leibniz’s extensive discussions of substance in his correspondences with De Volder and Des Bosses.
One of Leibniz’s most important discussions of substance occurs in his ten-year correspondence (1706–1716) with Bartholomew Des Bosses.A Jesuit theologian, Des Bosses approached Leibniz with the bold plan of providing a systematic presentation of Leibniz’s philosophy that would show it to be consistent with the doctrines of Aristotle and the dogmas of the Catholic Church. Des Bosses was ultimately unable to deliver on this grand plan, which Leibniz encouraged; but their preliminary discussions were philosophically rich and illuminating. One of the focal points for their discussions of substance was the question of whether Leibniz’s theory of substance was compatible with the Catholic doctrine of transubstantiation. Given this explicitly conciliatory context, these discussions should provide an ideal test case for McDonough’s interpretive framework.
The correspondence with Des Bosses is one of the few places where Leibniz discusses the theory of monads in considerable detail. In keeping with his standard usage in later texts, Leibniz uses the term “monad” to designate a simple, immaterial, mind like substance—a per se unity and principle of activity, possessing the modifications of perception and appetite. Leibniz is very clear in stating that monads are substances. As he says on 31 July 1709, for example, “monads can subsist by themselves, for they are substances” (LR: 136–137). Leibniz gives no indication anywhere in the correspondence that this is a position he is willing to call into question. In other words, Leibniz says nothing that suggests that monads meet all the philosophical requirements for being substances but might nonetheless fail to be substances, as McDonough’s interpretation holds. Rather, given that monads satisfy all the philosophical requirements for being substances, they must be substances.
The most contentious question that Leibniz considers with Des Bosses is whether corporeal substances, conceived as unions of a monad with an organic body, are also substances in the strict sense. Leibniz consistently affirms throughout the correspondence that monads alone—a dominant monad and the aggregate of monads constituting its organic body—cannot constitute such a substance. He writes on 26 May 1712, for example: “And Monads do not constitute a complete composite substance, since they do not make a per se unity, but a mere aggregate, unless some substantial bond is added” (LR: 242–243). It is crucial to note the reason Leibniz does not think that monads alone can constitute a composite substance—it is that any true substance must have per se unity, and a dominant monad and its organic body is an aggregate, strictly speaking, not a per se unity.
In the text just quoted Leibniz does mention a condition according to which a corporeal substance could be a substance, strictly speaking. This can only occur if a “substantial bond [vinculum substantiale]” is added. The hypothesis of the vinculum is a much-discussed addendum to the theory of monads that Leibniz considers in the Des Bosses correspondence.The details of Leibniz’s conception of the vinculum undergo some changes throughout the correspondence, but the function of the vinculum is clear: it transforms something that is an aggregate—a dominant monad and its organic body—into a per se unity. This is another place where Leibniz’s stated view diverges from the position that McDonough attributes to him. On McDonough’s reading, Leibniz is committed to organisms satisfying all the philosophical requirements for being substances, including being per se unities. He takes Leibniz to entertain different views on how organisms are per se unities, but the idea that organisms are—in some way or another—per se unities, is a thesis to which Leibniz is consistently committed. The view that Leibniz presents in the correspondence with Des Bosses, however, is that the only way for organisms to be per se unities is if there are substantial bonds. If there are no substantial bonds, then organisms are not per se unities, and cannot be substances, strictly speaking.
Although it is beyond dispute that Leibniz explores the hypothesis of the substantial bond in the correspondence with Des Bosses, the extent to which he is willing to endorse the hypothesis himself is not immediately clear. Some people have argued that Leibniz does, at least for a time, endorse the hypothesis (Garber 2009: 381). Others have argued that Leibniz is not committed to the theory himself. He only develops it as an addendum to the theory of monads that Des Bosses might want to affirm.Recall that Des Bosses is trying to synthesize Leibniz’s metaphysics with Aristotle and the doctrines of the Catholic Church. Des Bosses was concerned that transubstantiation could not be properly accounted for unless corporeal substances were substances, strictly speaking. As a Lutheran, Leibniz need not account for the doctrine of transubstantiation. Nevertheless, he works with Des Bosses to develop an addendum to the theory of monads that might be more attractive to Catholics. I favor an interpretation along these lines, but I will not try to make that case in this paper.
Regardless of how one answers the contentious question about Leibniz’s commitment to the theory of substantial bonds, it is clear that there is a theological dimension to Leibniz’s treatment of substance in the Des Bosses correspondence. It is important to consider just what this theological dimension involves. Consider these remarks in Leibniz’s 15 February 1712 letter:
 If corporeal substance is something real over and above monads … we shall have to say that corporeal substance consists in a certain union, or rather in a real unifier superadded to monads by God … Thus, one of two things must be said: either bodies are mere phenomena, and so extension will be only a phenomenon, and monads alone will be real, but with a union supplied by the operation of the perceiving soul on the phenomenon; or, if faith drives us to corporeal substances, this substance consists in that unifying reality, which adds something absolute (and therefore substantial) … to the things to be unified. (LR: 226–227)
Leibniz’s remark in this passage about faith driving one to corporeal substances might initially seem suggestive of McDonough’s account, but it diverges from that account in two key respects. First, according to McDonough’s analysis, theological considerations might lead one to hold that corporeal substances are the unique true substances at the expense of immaterial simples. The view that Leibniz suggests here, however, is one according to which both organisms and immaterial simples are substances, strictly speaking. The substancehood of organisms does not come at the expense of the substancehood of monads.
The second key difference concerns the point at which theological considerations enter into Leibniz’s analysis. On McDonough’s view, one’s sectarian religious commitments come into play after a purely philosophical analysis reveals the candidates for substances. While both immaterial simples and organisms satisfy all the philosophical requirements for being substances, a person’s extra-philosophical views might nonetheless lead her to take one of these two candidates as the true substances. What Leibniz suggests here, however, is that without a substantial bond an organism does not satisfy the philosophical requirement of being a per se unity. Faith is not brought in as a consideration over and above the philosophical considerations; rather, it is brought in as a motivation for endorsing a hypothesis that would allow organisms to satisfy the requirement of being a per se unity. If it satisfies this—and the other—philosophical requirements, then it will be a substance in the strict sense. Contrary to McDonough’s suggestion, there is no indication that an organism could satisfy all the philosophical requirements for being a substance and still fail to be a substance.
Let us turn now to Leibniz’s correspondence with De Volder. I will keep my criticisms brief, as I will be discussing this correspondence at greater length when I present my positive view. Let me begin by noting an important difference between Des Bosses and De Volder. Unlike Des Bosses, De Volder did not invoke extra-philosophical concerns in his discussions of substance. He does not, for example, explicitly consider how well various theses about substance cohere with doctrines of revealed theology. The sorts of considerations he emphasizes can reasonably be described as "purely philosophical."From the perspective of McDonough's interpretation, De Volder seems like the sort of interlocutor to whom he would be willing to reveal his philosophical analysis of substance. At first glance, at least, some texts seem to line up well with this suggestion. Consider this famous passage from Leibniz’s 20 June 1703 letter:
 I therefore distinguish: (1) the primitive entelechy, i.e. the soul; (2) matter, namely primary matter, i.e., primitive passive power; (3) the monad completed by these two things; (4) the mass, i.e., the secondary matter, i.e., the organic machine, for which innumerable subordinate monads come together; and (5) the animal, i.e. the corporeal substance, which the monad dominating in the machine makes one. (PL: 264–265)
Few passages in Leibniz’s corpus have been discussed more than this text, which is often described as laying out a five-fold ontological scheme.Taken in isolation from the rest of the correspondence, I think that this text can be read in line with McDonough's interpretation.
It is crucial to note, however, that some of Leibniz's subsequent letters provide increasingly explicit affirmations of an idealist position. Consider the following remarks from two of Leibniz's later letters:
 Bodies, which are commonly taken for substances, are nothing but real phenomena, and are no more substances than perihelia or rainbows, and this is not something that is overturned by touch any more than by sight. A monad alone is a substance; a body is substances, not a substance. And the difficulties concerning the composition of the continuum, and others of this kind, cannot be made to disappear any other way. (21 January 1704, PL: 286–287)
 Indeed, considering the matter carefully, it should be said that there is nothing in things except simple substances and in them perception and appetite. Moreover, matter and motion are not so much substances or things as the phenomena of perceivers, the reality of which is located in the harmony of perceivers with themselves (at different times) and with other perceivers. (30 June 1704, PL: 306–307)
In the 21 January 1704 letter Leibniz claims that "a monad alone is a substance." It is clear from the context of this passage that Leibniz is using the term "monad" in his standard sense indicating a simple immaterial mind-like entity possessing the properties of perception and appetite. He is speaking of the same entities when he states in the 30 June 1704 letter that "there is nothing in things except simple substances and in them perception and appetite." Quite plausibly, these texts have been taken to affirm an ontology according to which monads are the only substances, strictly speaking.
McDonough himself cites the 30 June 1704 letter as a place where Leibniz appears to affirm an idealist ontology (2013: 7). It is difficult, however, for McDonough to account for this sort of text in the context of the correspondence with De Volder. Recall that on McDonough's interpretation, Leibniz provides an idealist slant to his ontology when he thinks the extra-philosophical commitments of his interlocutor would lead that person to affirm an ontology of immaterial simples. But De Volder does not, so far as I have been able to discern, have any extra-philosophical commitments that would make this a sensible strategy to pursue. Unlike Des Bosses, the considerations that De Volder invokes are straightforwardly philosophical. And his philosophical tendencies do not make him particularly well disposed towards an idealist ontology. Consider how De Volder responds to Leibniz's 30 June 1704 letter:
 … I noticed many entirely new and unexpected things in your last letter. For instance, I have kept asking where the force of corporeal substance arises from. It now seems to me that you do away with bodies altogether, inasmuch as you place them only in appearances, and that you substitute forces alone for things, and not even corporeal forces but “perception and appetite”. (14 November 1704 letter from De Volder, PL: 312–313)
If Leibniz were committed to the view that McDonough attributes to him, we would expect him to emphasize to De Volder his philosophical analysis of substance according to which both immaterial simples and organisms exist and satisfy all of the philosophical requirements for being true substances. As we have seen, an earlier text in their correspondence can be read as presenting such a view; but Leibniz follows up that statement with explicit affirmations of an idealist position.When Leibniz sees that De Volder is not favorably disposed towards this position, he does not soften his affirmations of idealism. While he does try to explain that he does not "do away with bodies altogether," he does not suggest that any entities other than monads qualify as substances, strictly speaking.It is fair to conclude, I think, that McDonough's interpretive framework cannot make good sense of Leibniz's ontological pronouncements in the De Volder correspondence. We have already seen that it cannot make good sense of the remarks on substance in the Des Bosses correspondence either. An interpretation of Leibniz's theory of substance that cannot make good sense of the De Volder and Des Bosses correspondences is not tenable, in my opinion.
Let me temper this negative verdict by saying that McDonough is quite right to insist that no “straightforward" or “surface reading” of Leibniz's many remarks on substance can yield a single consistent theory of substance. I think he is also right to insist that Leibniz can take organisms to be real and exist even if they are not substances, strictly speaking. Finally, I agree with him that it is important to pay careful attention to Leibniz's use of rhetorical strategies in his discussions of fundamental ontology. In the next section of the paper I will develop an alternative framework for understanding Leibniz’s rhetorical strategies. I will then show how this framework can help us make sense of some of Leibniz’s most significant remarks on fundamental ontology.
4. Pedagogical Exoteric Writing
The aim of this section is to provide an overview of Leibniz’s views on exoteric writing. As we will see, Leibniz’s views on this topic should inform our readings of the texts at the center of the debate between idealist, realist, and developmental interpretations of Leibniz’s ontology. Let us begin by considering the following excerpt from a 1702 letter to Pierre Bayle:
 … I should not be in too much of a hurry to publish what I have written, the point of which was only to provide some clarification for you, sir, and for some other people, so as to receive the same in return. For I write not so much to make an impression as to investigate the truth, which it is often useless, and even harmful, to publish—on account of the uninitiated, who are incapable of appreciating it, and quite capable of taking it the wrong way (G 3: 66–67; WF: 127).
At first glance one might take this text to suggest Bertrand Russell's view according to which Leibniz has two radically different philosophies, a philosophical fairy tale for the public and his true philosophy, derived from austere logical principles, which he mostly kept to himself (1945: 281). Leibniz's view is far subtler than this crude characterization suggests, however. His point, as additional texts will make clear, is that it is a mistake to straightforwardly present many of his views to his readers.
It is important to see why Leibniz thought this would be a mistake. We can start by considering some general remarks that he makes in the preface to the Nouveaux Essais about the differences between his philosophy and the philosophy of Locke:
 … although the author of the Essay says hundreds of fine things which I applaud, our systems are very different. His is closer to Aristotle and mine to Plato, although each of us parts company at many points from the teachings of both of these ancient writers. He is more popular [populaire] whereas I am sometimes forced to be a little more esoteric [acroamatique] and abstract—which is no advantage for me, particularly when writing in a living language. (NE: 48)
In this text Leibniz is drawing attention to the fact that the content of his philosophy is less “popular” than Locke’s. Locke’s philosophy is popular in the sense that much of it accords with the deliverances of the senses and so-called “common sense.” Leibniz’s philosophy, in contrast, is often very abstract and far removed from ordinary opinions. This point is emphasized in a revealing 1702 letter to Sophie Charlotte, where he claims that “besides the sensible and the imaginable, there is that which is only intelligible, the object of the understanding alone” (G 6: 501; AG: 188, emphasis in original). In his elaboration of the distinction between the sensible and the intelligible he writes:
 … what the ancient Platonists have remarked is very true, and very worthy of consideration, that the existence of intelligible things, and particularly of this I who thinks and is called mind or soul, is incomparably more certain than the existence of sensible things, and thus, that it would not be impossible, speaking with metaphysical rigor, that, at bottom, there should only be these intelligible substances, and that sensible things should only be appearances. However, our lack of attention lets us take sensible things for the only true things. (G 6: 502–503; AG: 189)
Leibniz thinks that there is a significant gap between the way things are according to the true metaphysics and the way people generally take things to be. Many people uncritically assume that the only things that exist are things that can in some way be sensed. Leibniz, in contrast, thinks that at least some substances exist that can be understood through the pure intellect, but which cannot be sensed at all. Indeed, in this passage he concedes the possibility of an idealist ontology consisting exclusively of intelligible substances (the extent to which he is willing to embrace such an ontology is not clear from the passage alone). One can thus begin to see why Leibniz would take himself to be at a significant disadvantage when it comes to presenting his metaphysics to the general public. Readers are far more likely to find favor with a philosophy such as Locke’s that is grounded in empiricist principles.
Because of this gulf between people's ordinary sense-based views and Leibniz’s intelligible principles, he thought it would be useless and harmful to straightforwardly present the content of his metaphysics to his interlocutors. As he explains to Charles Hugony, “some of my views cannot be presented in a straightforward manner [ne peut donner cruement], since people are liable to misunderstand them, not in relation to religion, which is strongly supported, but in relation to the senses” (6 November 1710, G 3: 680). Similarly, in a letter to Simon Foucher from the late 1680s Leibniz notes that some of the core concepts of metaphysics are those of “cause, effect, change, action, time, where I find that the truth is very different from what one imagines” (G 1: 391). After setting forth several of his more controversial views he cautions Foucher as follows: “it is not appropriate for these sorts of considerations to be seen by everyone, and the vulgar would not be able to understand it at all before having the mind prepared” (G 1: 392, emphasis added).
How does Leibniz address the problem he faces in trying to present the esoteric content of his philosophy to others? His remark to Foucher about “preparing the mind” provides a clue. Additional clues can be found in some unpublished remarks appended to metaphysical notes that Leibniz wrote around 1676.
 Metaphysics should be written with accurate definitions and demonstrations, but nothing should be demonstrated in it that conflicts too much with received opinions [sententiis receptis]. For thus this metaphysics will be able to be received. If it is once approved, then afterwards, if any examine it more profoundly, they will draw the necessary consequences themselves. Besides this, one can, as a separate undertaking, show these people later the way of reasoning about these things. In this metaphysics, it will be useful for there to be added here and there the authoritative utterances of great men, who have reasoned in a similar way; especially when these utterances contain something that seems to have some possible relevance to the illustration of a view (A VI.iii: 573; DSR: 95).
Perhaps the most striking claim in this remarkable text is Leibniz’s admission that it would be a mistake to present metaphysical views that “conflict too much with received opinions.” It is suggested, though not explicitly stated, that at least some of Leibniz’s own views do conflict significantly with received opinions. His remarks imply that if he were to set forth views that conflicted too much with received opinions then those views would be unlikely to be accepted. Leibniz does not explain why he thinks this in the present text. The view that he presents in some of the later texts that we have already examined is that this is because many people would regard such views as absurd, summarily reject them, or misunderstand them entirely—particularly if they concern purely intelligible things that cannot be known through the senses. Leibniz suggests a strategy of selective omission as a way of dealing with these problems. It is crucial to recognize that this in not done in order to permanently hide the controversial features of his metaphysics from the public. Selective omission is part of a longer-term strategy of carefully preparing his readers to understand his most esoteric doctrines. In some cases he thinks that sympathetic readers who study his texts “more profoundly” will be able to infer the esoteric conclusions themselves. Leibniz also suggests a complementary strategy of supplementation. In subsequent works he can reveal the deeper meaning of the text, or explicitly draw the conclusions that may have only been implicit in it. Taken collectively, Leibniz’s remarks in this passage suggest that the primary function of exoteric texts is pedagogical. They are carefully crafted to serve as intellectual stepping-stones that will enable his readers to gradually move from received opinions to esoteric doctrine.
As text  suggests—and additional texts confirm—Leibniz utilized a range of rhetorical strategies when he composed exoteric texts. Here is a brief summary of four key strategies that will be particularly important for the main theme of this paper:
Selective Omission: he omits the features of his philosophy that are furthest removed from received opinions. In some cases it is possible to infer the omitted views from a careful reading of the exoteric text; in other cases a supplementary work is required.
Surface Reading vs. Deep Reading: the text admits of an intuitive surface reading that can make his views seem closer to received opinions than they actually are. The texts are also compatible with a more rigorous but less straightforward reading. It may or may not be possible to grasp the deeper meaning on the basis of the exoteric text alone.
Familiar Language: he uses terminology that will be familiar to his interlocutors despite the fact that the terms do not have the same meaning within his system. This allows Leibniz to initially present his views in a way that makes them seem closer to the views of his interlocutors than they actually are.
Supplementation: In a subsequent letter or text Leibniz more explicitly reveals his views to his interlocutors. He might introduce a feature of his system that was previously selectively omitted, explain the deeper meaning of a claim that was previously made, or explain more precisely how he is using technical terms that were left undefined in a prior text. Leibniz uses this strategy most frequently in his correspondences. It is important to recognize, however, that Leibniz characterized his published essays and the Essais de Théodicée as works that were designed to prepare his readers for a text that would provide a more rigorous presentation of his views.
It is important to recognize that Leibniz’s use of the rhetorical strategies listed above is largely implicit. This is not surprising. In most cases being fully explicit about one’s use of exoteric strategies would undermine the whole point of pedagogical exoteric writing. It is thus easy to miss Leibniz’s use of exoteric strategies. This can lead us to take Leibniz’s statements on a topic in different contexts to be contradictory when in reality the difference is only in the mode of exposition.
How can one determine whether an esoteric/exoteric interpretation is appropriate? Ideally one should (a) look at a range of texts where Leibniz discusses the issue (the more the better), (b) approach the texts with an awareness of the function of exoteric writing, and the specific exoteric strategies that he employs, (c) take note of the philosophical views of his interlocutors or target audience, (d) look for indications that Leibniz’s view on the issue is far removed from the opinions of his target audience, and (e) look out for subtle linguistic clues that suggest an exoteric or esoteric context. Depending on the results of (a)–(e), it might be possible to develop a principled esoteric/exoteric interpretive hypothesis.
5. Idealism and Pedagogical Exoteric Writing
I think that Leibniz was firmly committed to an idealist ontology from 1695 (possibly earlier) until his death. I think that the texts that people have taken to indicate a commitment to a non-idealist ontology are actually consistent with a commitment to idealism. A full defense of this thesis would require the careful, properly contextualized, evaluation of all of Leibniz’s remarks on fundamental ontology. My aims in the remainder of this paper are considerably more limited. Utilizing Leibniz’s remarks on pedagogical exoteric writing, I will sketch an interpretive framework for making sense of Leibniz’s remarks on fundamental ontology and explain how Leibniz uses pedagogical exoteric writing in his correspondence with De Volder. Before proceeding, one qualification is in order. In claiming that Leibniz is firmly committed to idealism during this period, I am not defending the position that Leibniz had worked out every detail of his ontological scheme. On my reading Leibniz took bodies and organisms to be “real” and “exist” despite the fact that they are not substances (there are degrees of reality in his ontology). He might not have worked out all the details of how to properly characterize the degree of reality that bodies possess.He remains steadfast, however, in his commitment to the view that only immaterial simples are substances, strictly speaking.
We have seen that Leibniz thinks that features of his metaphysics cannot be straightforwardly presented to people because they are likely to be regarded as absurd and unintelligible, leading to their swift rejection. Let me distinguish two problems that directly relate to Leibniz’s presentation of an idealist theory. Both of them stem from an over-reliance on the senses:
- (i) People are not in the habit of attempting to conceive of simple, immaterial, mind-like substances.
- (ii) People think it is obvious that bodies must be substances.
Recall Leibniz’s 1702 letter to Sophie Charlotte, which is suggestive of both of these points: “it would not be impossible, speaking with metaphysical rigor, that, at bottom, there should only be these intelligible substances, and that sensible things should only be appearances. However, our lack of attention lets us take sensible things for the only true things” (G 6: 502–503; AG: 189).
Consider also what he tells Des Bosses in 1715:
 But it is another question whether bodies are substances. For even if bodies were not substances, nonetheless all men will be inclined to judge that bodies are substances, just as they are all inclined to judge that the Earth is at rest, even though it is really in motion. (19 August 1715, LR: 347)
While both (i) and (ii) are barriers that make it difficult for people to understand and accept an idealist theory, (ii) is the biggest problem. One can see immediately that if idealism is true then bodies are not substances. One is likely to infer from this that:
- (iii) Bodies are purely illusory.
Leibniz plausibly thinks that readers who infer (iii) are likely to reject idealism in short order. This is, in fact, what happens in the De Volder correspondence. Recall De Volder’s response when Leibniz presented his idealist ontology in no uncertain terms. De Volder claimed to notice “many entirely new and unexpected things” in the letter. He suggested that Leibniz now seemed to “do away with bodies altogether, inasmuch as you place them only in appearances.”In thinking that Leibniz is doing away with bodies altogether, De Volder is making precisely the mistake that Leibniz hoped to have his readers avoid. Leibniz’s response to De Volder is particularly instructive in this regard:
 You say that you noticed many surprising things in my most recent letters. But you will perhaps observe that the same views had already been suggested in previous letters, and only prejudice has prevented you from coming to this point some time ago and at long last stopping your search for substances and for the source of forces where it isn’t to be found. And so, I was forced to impress certain of my views on you more explicitly, and to respond, if not to what you asked, at least to what you should have been asking. (1705 letter to De Volder, G 2: 275; AG: 181)
Leibniz’s confession in this passage shows that he was not fully revealing his views to De Volder in his earlier letters. What he was not revealing, this text strongly suggests, is his fully idealist ontology. But how does Leibniz try to engage readers like De Volder without fully revealing his idealist ontology? Let us begin by distinguishing the following two theses:
- (iv) Simple, immaterial, mind-like substances exist.
- (v) Simple, immaterial, mind-like substances are the only true substances.
With this distinction in mind, let me offer the following interpretive hypothesis: Leibniz recognized that if he straightforwardly presented (v) to his readers, then they would most likely infer (iii); and given (ii), they would most likely regard the theory as absurd and summarily reject it. To combat this danger, Leibniz is careful to introduce (iv) without explicitly endorsing (v); that is, he attempts to introduce the idea that simple immaterial substances exist without adding the further more controversial claim that the only true substances are simple immaterial mind-like ones. Leibniz seeks to help people properly conceive of these purely intelligible entities, to recognize some of the philosophical advantages of positing such entities, and to see the weaknesses of competing views. Leibniz hopes that his readers will then be able to infer his idealist position from the subtle suggestions that he leaves in his texts. This occurs both in his published pieces and in his correspondences. In correspondences he sometimes introduces more explicit statements of idealism as the correspondences progress. Leibniz’s remarks in text  suggest that he utilized just this sort of progression in the correspondence with De Volder. Let us now consider how this is supposed to work in detail.
We must start at the beginning of the correspondence—before the beginning, in fact. In a 21 November 1698 letter to Leibniz’s friend Johann Bernoulli, De Volder mentioned some questions he had about Leibniz’s critique of the Cartesian conception of force (PL: 20–21). He was particularly interested in Leibniz’s remarks about the activity of substance in a short Acta eruditorum article. He hoped that Leibniz could provide an a priori demonstration of the thesis that every substance is active. This would conclusively refute the causal theory of Occasionalism (roughly the thesis that God is the only genuine causal agent), with which De Volder had little sympathy. It is crucial to note here that De Volder was looking for a demonstration of the activity of corporeal substances. De Volder was, unsurprisingly, one of the many people who took bodies to be substances.
Leibniz did not attempt to provide an a priori demonstration of the activity of substances in his first letter to De Volder. Responding rather cautiously to De Volder’s inquiry, he wrote:
 And while it seems to me that I have long been advancing at a walking pace, you easily understand on account of your good sense how difficult it is to demonstrate clearly things that are sufficiently far removed from the common understanding and the prejudices of most people, and to present them so that they are safe from the treachery of ignorant men and often from the censure of the spiteful (27 December 1698, PL: 46–47).
Leibniz’s remarks make it seem like he was concerned with the way that other people would receive his ideas. However, he was aware that De Volder was looking for an account of the activity of corporeal substances, and thus that De Volder’s views were in keeping with the prejudices of most people about the substantiality of bodies. Leibniz confides as much to Bernoulli several months later when he says that De Volder’s “objections seem to me mostly to originate from prejudices. His notion of extension is very different from mine. It is the same with substance and many others” (6 July 1699, PL: 110–111). These are crucial points to keep in mind, for as we saw in the last section, whenever someone’s metaphysical views are far removed from Leibniz’s views, he is likely to use a range of pedagogical strategies to help them gradually progress from their starting point to a position where they are capable of appreciating the true metaphysics.
In his 27 December 1698 letter, Leibniz told De Volder that he had written some things about substance in recent journal articles. De Volder read those articles and responded as follows:
 I have read through what you say in the Journal des Savants, the Acta eruditorum, and the Histoire des ouvrages des Savants concerning the nature of substances, the communication between them, and their forces. But for the most part, I see it treated so briefly and concisely that I dare not persuade myself that I have understood what you mean. You seem to me to deny that extension is a substance [Extensionem mihi videris negare substantiam] … (18 February 1699 letter from De Volder, PL: 60–61)
It is understandable that De Volder was unsure about Leibniz’s views on substance after reading these articles. Although I will not discuss them in this paper, Leibniz’s treatments of substance in those texts were, as De Volder noted, brief. They were also less than fully explicit about which entities Leibniz took to be substances, strictly speaking. This should not be surprising if, as I suggested above, Leibniz is careful to introduce his readers to (iv) without explicitly endorsing (v). What would be surprising is if any of these texts provided unambiguous affirmations of an idealist ontology (v). While none of these texts explicitly affirmed (v), they did contain remarks that pointed in its direction.Hence De Volder’s remark about Leibniz seeming to deny that extension is a substance. Given his commitment to the substantiality of body, it is not surprising that he followed this remark with a brief defense of the idea that extension is a substance (PL: 60–61).
In his next letter Leibniz responds—again rather cautiously—by explaining why he does “not think that there is a substance constituted from extension alone …” (3 April 1699, PL: 73). The details of Leibniz’s response can be omitted here; I only want to emphasize that, while suggestive, his remarks fell well short of an explicit endorsement of idealism. De Volder, in his next letter, was not convinced by Leibniz’s criticisms of the notion of extension as he conceived of it, and continued to insist on its substantiality (13 May 1699, PL: 88–89).
Leibniz, in his letter of 6 July 1699, did not express surprise in finding out that he had not convinced De Volder with his previous letter: “Few men can say things—even true things—in such a way that they are accepted immediately, and the truth does not always seem like the truth. Besides, we all have prejudices that are very hard to eradicate” (PL: 96–97). Leibniz goes on to say, “I do not know whether it is advisable to immerse ourselves in more obscure issues—such as those concerning the nature of substance and extension, where I see that our notions are different—before we come to an agreement about the measure of force” (PL: 98–99).
Leibniz’s stated reason for temporarily tabling the discussion of the nature of substance was that it would be better not to try to discuss too many things at once (6 July 1699, PL: 98–99). The deeper reason, I believe, is that because Leibniz’s own views on the nature of substance and extension are so different from De Volder’s, he thought it would be unwise to focus on these differences at this point in the correspondence. Leibniz had not yet done much to reveal exactly what his views were, and he was worried that De Volder would reject them if they were presented to him in a straightforward manner. Instead, he decided to focus on their disagreements about the measure of force. Leibniz thought that if he could get De Volder to think carefully enough about the notion of force, this would allow him (De Volder) to infer more about Leibniz’s views on the nature of substance and extension. This comes out more explicitly in one of Leibniz’s subsequent letters. In a postscript to the first letter in which he introduces the notion of a monad, he writes:
 P.S. I have learned from our friend Mr. Bernoulli that it seems more important to you that light be shed on the activity of substance than that the force of bodies be measured. I agree, and I approve of your judgment. But nonetheless, it has always seemed to me that this is the gate through which to pass to the true metaphysics. The soul is gradually freed from the false notions of the populace, and even of the Cartesians, concerning matter and motion and corporeal substance, when it has come to understand that the rules of force and action cannot be derived from these notions, and that now one must either take refuge in Deus ex machina or understand there to be something higher in bodies. But if the mind is led unprepared into this holy of holies, where the completely unexpected nature of substance and body can be viewed from its origins, then there is a fear that its darkness will be overwhelmed by the excessive light. (1 September 1699, PL: 130–131)
When read against the background of Leibniz’s views on pedagogical exoteric writing, and the inherent challenges of setting forth an idealist ontology, this text makes a great deal of sense. The soul needs to be gradually freed from false notions concerning matter, motion, and corporeal substance in order to pass to the true metaphysics. Thinking carefully about the rules of force and action is one of the best ways to do this.This is, in effect, what Leibniz had been trying to do with De Volder in their correspondence to this point. To simply present the true metaphysics to people—without adequately preparing their minds—would be a mistake because they would be incapable of understanding the unexpected nature of substance and body. The mind must be gradually led from the darkness into the light.
Notice that in text  Leibniz makes no explicit reference to an idealist ontology. In speaking of “something higher in bodies” he is almost certainly speaking of monads; but as we have seen there is a big difference between (iv) and (v)—that is, between claiming that simple substances exist and claiming that the only true substances are simple substances. At this point in the correspondence Leibniz has just started to introduce (iv) to De Volder; he has not made any comments that explicitly commit him to (v). In his 1 September 1699 letter Leibniz’s reference to monads was very brief. He simply said: “I understand there to be a true unity (not merely a sensible one), i.e., a monad, where there is something in which there are not many substances” (PL: 126–127, emphasis in original). Though he did not elaborate on the nature of this entity, he did emphasize that the notion of a monad is an intelligible one as opposed to a sensible one:
 But we are so made and predisposed that, although we properly distinguish intelligible things from imaginable things in theory, so to speak, and publicly declare that we distinguish them, nevertheless, in practice we do not observe this and we take those things that are not imaginable to be almost nothing. (PL: 128–129)
Leibniz hoped that these remarks, in conjunction with the postscript to the letter (cited above), and what he had written in his previous letters, would help De Volder recognize the existence of purely intelligible monads. He intimates that De Volder will find this conception of substance, along with his account of body, “completely unexpected,” which is why he chooses not to fully elaborate his view at this point in the correspondence.
De Volder continues to find Leibniz’s criticisms of his conception of substance unconvincing. He is also frustrated by the fact that Leibniz seems to be holding back on the details of his own conception of substance. This leads him to register the following complaint:
 If only it had pleased you to include your notion of substance. For if I had discerned your intentions better, I would have understood your objections more clearly. And at the same time, by combining both definitions, I think I would have uncovered what is lacking in my definition more easily. (13 February 1701, PL: 204–205)
In his subsequent letter Leibniz continued to criticize De Volder's conception of substance. He made a few suggestive remarks about his positive views, but did not present anything like a full account. In this regard not a lot had changed from his previous letters to De Volder. What is particularly striking is the way that he then responded to De Volder's complaint: “As for my own notion of substance, I would prefer for it to arise from our mutual considerations (and certainly I think that a good start has been made), rather than for it to be produced by me alone and as if it were forced upon you” (6 July 1701, PL: 210–211). Leibniz’s remark here is of great importance for the interpreter of his correspondence with De Volder. He is explicit in saying that he will not be simply laying out his account of substance to De Volder. It would be better, he thinks, for De Volder to try to infer the position himself based on their “mutual considerations.” What we should expect in subsequent letters is for Leibniz to include remarks that, while not fully explicit, would enable De Volder to infer Leibniz’s views on fundamental ontology for himself.
Progress was slow in the discussions of substance that took place in the next few letters of the correspondence. As he had done in previous letters, Leibniz provided critiques of De Volder’s conception of substance without doing a great deal to explain his own view. In Leibniz’s 27 December 1701 letter, for example, he suggested that De Volder’s account of substance in general would imply that only God is a substance.
 I admit that you are within your rights to understand the word substance so that God alone is a substance and other things are called something else. But it is my intention to look for a notion that will apply to others as well, and that agrees with ordinary ways of speaking, according to which you and I and others are counted as substances [Sed mihi animus est notionem quaerere quae conveniat caeteris quoque, et hominum usui consentiat, quo Tu, ille, ego substantiis accensemur]. (PL: 222–223)
Leibniz is well aware that his metaphysics diverges considerably from received opinions, but he does not want to depart from ordinary ways of speaking any more than he has to. Recall that Leibniz sometimes uses familiar language as a pedagogical strategy for presenting his views to people. This can be a useful strategy insofar as it makes his views seem not so far removed from received opinions, but it can also lead to misunderstandings if his interlocutors assume that he is using terms in exactly the same sense that they are. In the present case Leibniz says it is important to preserve the ordinary way of speaking according to which “you and I and others” count as substances. This provides some degree of contrast between Leibniz’s view and a view according to which God is the only substance, but it is ambiguous on the central point of dispute between idealist and so-called “realist” interpretations of Leibniz’s mature ontology. As McDonough emphasized in his interpretation, the claim that “you and I and others” are substances could mean that you and I are simple, immaterial, mind-like substances; or it could mean that you and I are organic unities.
Given that Leibniz did not elaborate on the precise sense in which you and I and others are substances, it is not entirely surprising that De Volder took Leibniz to be using the terms “monad” and “corporeal substance” synonymously in his next letter.One of the primary reasons Leibniz presents the famous “five-fold ontological scheme” in his next letter is to explain to De Volder how he distinguishes monads and corporeal substances. Let us quote this passage in full once more:
 I therefore distinguish: (1) the primitive entelechy, i.e. the soul; (2) matter, namely primary matter, i.e., primitive passive power; (3) the monad completed by these two things; (4) the mass, i.e., the secondary matter, i.e., the organic machine, for which innumerable subordinate monads come together; and (5) the animal, i.e. the corporeal substance, which the monad dominating in the machine makes one (20 June 1703, PL: 264–265).
There is no question that Leibniz’s 20 June 1703 letter marks a new phase in their correspondence. Exactly what that phase is, however, needs to be carefully considered. Many commentators have taken Leibniz to be straightforwardly laying out the structure of his ontology in this passage. If that were true, then Leibniz’s characterization of the “animal” as a “corporeal substance” would seem to be strong evidence in favor of the thesis that animals are substances, and thus that Leibniz was not committed to idealism when this text was written.However, if we look at this passage in the context of the entire 20 June 1703 letter and the correspondence as a whole, we will see that this passage admits of a perfectly natural idealist reading.
Recall that Leibniz told De Volder in an earlier letter that he wanted his conception of substance to emerge from mutual considerations, so that De Volder could “discover” his conception of substance rather than having it forced upon him. Leibniz did not abandon that strategy in the 20 June 1703 letter. The letter builds on De Volder’s previous letter by distinguishing between monads and corporeal substances and more carefully explaining the characteristics of monads than he had done before. It thus provides a clear affirmation of (iv). Getting De Volder to move from (iv) to (v) is a subsequent step. This subsequent step is perhaps the most difficult; for as we have seen, there is a tendency to move from (v) to the conclusion that bodies are purely illusory (iii). I believe that one of Leibniz’s strategies for avoiding this fallacious inference is to initially use language that describes organisms and bodies in ways that his interlocutors are likely to read as according more reality to these entities than they possess in Leibniz’s considered ontological scheme. This is what I take Leibniz to be doing in text . He characterizes animals as “corporeal substances” even though they are not substances, strictly speaking.
Leibniz’s strategy for keeping his interlocutors from erroneously inferring (iii) is something of a double edged sword. While it can help his interlocutors recognize the important role that bodies and organisms play in his ontology, it can also make it difficult for them to move from (iv) to (v). Nevertheless, Leibniz seems to think that this is a better overall pedagogical strategy because when they do infer (v), they will be able to see that bodies and organisms have a place in the true ontology despite not being substances, strictly speaking. This, combined with an awareness of the weaknesses of rival theories of substance and the virtues of postulating immaterial mind-like substances, will help them properly understand—and hopefully endorse—Leibniz’s idealist ontology.
While Leibniz’s 20 June 1703 letter does contain remarks that, taken at face value, can suggest that monads and organisms are substances, it also contains at least two passages that are suggestive of an idealist ontology. In the paragraph that follows the five-fold ontological scheme, Leibniz writes: “… since simple things alone are true things, the rest are only beings through aggregation, and therefore phenomena” (PL: 264–265). Given that Leibniz typically only characterizes monads as simples, this passage suggests that monads alone are true substances.
Secondly, shortly before presenting the five-fold ontological scheme, Leibniz claims: “I do not admit the action of individual substances on one another, since there appears to be no way by which a monad may influence a monad” (PL: 263). In this passage Leibniz denies that substances causally interact on the basis of his view that monads do not causally interact. If he thought that monads and animals were both substances, he would not be able to answer the general question about causal interaction merely by noting that monads do not causally interact. The fact that he does reason in this way suggests that only monads are substances, strictly speaking. I think Leibniz is hoping that De Volder himself will infer (v)—or at least raise it as a possibility—on the basis of these suggestive passages.
In his next letter De Volder does make some brief remarks on the topic of inter-substantial causation. He notes, among other things, the systematic connection between Leibniz’s denial of inter-substantial causation and other central theses in his metaphysics:
 For if I assume that the various things in this universe are substances and that they do not act on one another, the rest seems to follow without difficulty. Since experience teaches that change happens, but this cannot happen without action and passion, and, moreover, substances can neither act on nor be acted on by one another, it will be necessary that we either summon Deus ex machina with Malebranche or assume with you that in each substance there is a certain force of acting, and that it contains all its changes intrinsically, and that they unfold successively. (30 October 1703 letter from De Volder, PL: 270–271)
De Volder admits that he prefers Leibniz’s hypothesis to Malebranche’s, but he thinks that Leibniz is simply assuming that substances act, which begs the question against Malebranche. De Volder also admits that he still does “not even really understand what that which you call substance is” (PL: 270–271). De Volder’s letter shows that while he saw that Leibniz denied (finite) inter-substantial causation, he did not see the idealist implication of Leibniz’s remarks on inter-substantial causation in the 20 June 1703 letter.
Leibniz’s nuanced continuation of this discussion in his next letter deserves close attention. He writes:
 Without a doubt you have hit the nail on the head when you judge that, according to my view, what are truly called substances (i.e., monads, i.e., the perfect substantial unities from which everything else necessarily results) [cum judicas ex sententia mea veri nominis substantias (id est Monades seu perfectas Unitates substantiales ex quibus caetera omnia resultare necesse est)] have no influence on one another, and that we are of the opinion that anything that happens otherwise belongs to the phenomena (19 November 1703, PL: 274–275, emphasis added).
The most striking thing about this text is that Leibniz says that De Volder judged something that he did not, in fact, judge. While De Volder did see that there is no inter-substantial causation, he did not restrict this claim to monads, and he did not characterize monads as the “perfect substantial unities” that “are truly called substances.” What is going on here? Did Leibniz simply misread what De Volder had written? This is unlikely. I contend that Leibniz is more explicitly drawing ontological implication (v), which he had hoped De Volder would draw himself from the 20 June 1703 letter. We have seen that Leibniz hoped that his own conception of substance would emerge from mutual considerations. With De Volder failing to pick up on the intimations of idealism that he had been presenting in his letters, he decided to take a slightly different tack. He tried to make it seem like De Volder had discovered the view by himself. He hoped that De Volder would be more sympathetic to the view having “discovered” it this way than if he dogmatically forced his view upon him. Leibniz thought that with text  at his disposal De Volder would finally see that his “corporeal substances” are not truly called substances. Both bodies and animals exist, and they play important roles in the true metaphysics, but they are not substances, strictly speaking.
De Volder’s next letter suggests that he still did not see that Leibniz meant to be affirming (v). He did not directly comment on Leibniz’s fairly explicit affirmation of (v). And he was clearly not interested in affirming an idealist ontology. Referencing a different remark in Leibniz’s previous letter, he wrote: “When I said that the universe is perhaps only one substance, I was clearly speaking about the corporeal universe, in this way following the common opinion of almost everyone of regarding bodies as substances …” (5 January 1704, PL: 282–283). These remarks, and the letter as a whole, show two things about De Volder’s views that are important for our purposes. First, he clearly took the idea that bodies are substances to be a thesis that almost everyone affirmed. Second, he thought that this common opinion was correct. Despite Leibniz’s numerous criticisms of the Cartesian conception of extended substance—some of which De Volder conceded—he showed no willingness to explore the idea that bodies are not substances, strictly speaking. Perhaps Descartes and others were mistaken in certain details of their views about bodies, but they couldn’t have been wrong on the basic point that bodies are substances (or so De Volder seems to think).
Leibniz’s correspondence with De Volder enters a new phase in his next letter. In this (final) phase of the correspondence Leibniz is, for the most part, just straightforwardly presenting his metaphysics to De Volder. I have suggested that in his previous letters Leibniz’s remarks on fundamental ontology started out with subtle gestures in the direction of (v) and gradually progressed to a nearly explicit affirmation of the idealist thesis in his 19 November 1703 letter. In his 21 January 1704 letter, by contrast, Leibniz gets right to the point: “We should see whether anything can be settled between us about monads” (21 January 1704, PL: 84–85). He follows this with a detailed account of how the reality of bodies is grounded in the reality monads, concluding with a completely explicit affirmation of (v):
 Bodies, which are commonly taken for substances, are nothing but real phenomena, and are no more substances than perihelia or rainbows, and this is not something that is overturned by touch any more than by sight. A monad alone is a substance; a body is substances, not a substance [Monada solam esse substantiam, corpus substantias non substantiam]. And the difficulties concerning the composition of the continuum, and others of this kind, cannot be made to disappear any other way (21 January 1704, PL: 286–287, emphasis added).
De Volder did not respond immediately to this letter. Four months later, having still not received a letter, Leibniz confided to Bernoulli that he was worried that De Volder had taken badly some of the things he had written, remarking that “he [De Volder] seems to have been writing without paying enough attention and to have shown himself to be insufficiently teachable” (2 May 1704, PL: 294–295). Although he is concerned that he and De Volder “may be debating in vain,” he is not worried that De Volder might find a serious problem with his system: “Indeed, since I have had my reasoning on these matters well organized for some time, it is hard to think of anything that might cause difficulty” (PL: 292–295). These important remarks fit very well with the interpretation I have been developing in this paper. They do not at all suggest that Leibniz had changed his mind on the ontological status of animals between 20 June 1703 (the letter containing Leibniz’s so-called 5 fold ontological scheme) and 21 January 1704 (the letter containing his first completely explicit affirmation of an idealist ontology). That would not seem to be in keeping with the claim that he “had his reasoning on these matters organized for some time.” Leibniz’s remarks highlight the pedagogical character of the correspondence. They show that he had been trying to teach De Volder. Leibniz was attempting to free De Volder from the false notions of the populace and the Cartesians about matter and corporeal substance. As he suggested in an earlier letter, the goal was to help De Volder understand “the completely unexpected nature of substance and body.” Leibniz recognized that his pedagogical efforts had been largely for naught, as De Volder proved to be “insufficiently teachable.” In Leibniz’s mind, De Volder failed to see the force of his criticisms of the Cartesian conception of substance; he failed to see the force of his arguments for (iv); and he failed to pick up on Leibniz’s increasingly explicit intimations of (v). Leibniz’s final strategy for engaging De Volder was simply to lay out his views to him in a fully explicit manner, drawing the inferences that De Volder had not been able to see. The letter from Bernoulli quoted above shows that Leibniz was not particularly optimistic about De Volder’s ability to understand—let alone to affirm—the central tenets of his ontology. Nevertheless, he thought there was a chance that when his views were made fully explicit, De Volder would see the force of the considerations he had been presenting in his previous letters.
The remaining letters in the correspondence with De Volder confirmed Leibniz’s fears: De Volder could not be made to understand Leibniz’s monadological metaphysics. The following statement from Leibniz’s 30 June 1704 letter seems to be what finally led De Volder to see that Leibniz affirmed an idealist ontology:
 Indeed, considering the matter carefully, it should be said that there is nothing in things except simple substances and in them perception and appetite. Moreover, matter and motion are not so much substances or things as the phenomena of perceivers, the reality of which is located in the harmony of perceivers with themselves (at different times) and with other perceivers (PL: 306–307).
While De Volder had been looking for the source of corporeal forces, it now seemed to him that Leibniz had done away with corporeal forces altogether and replaced them with perception and appetite. Leibniz did not take himself to be doing away with bodies in text . He clearly thought that matter and motion have “reality;” they just have less reality than De Volder had been assuming. These nuances seem to have been lost on De Volder, who inferred (iii) in short order after seeing that Leibniz endorsed (v) and the thesis that matter and motion are “the phenomena of perceivers.”
We are now well placed to return to Leibniz’s revealing response to De Volder, which I cited at the beginning of this section:
 You say that you noticed many surprising things in my most recent letters. But you will perhaps observe that the same views had already been suggested [insinuata] in previous letters, and only prejudice has prevented you from coming to this point some time ago and at long last stopping your search for substances and for the source of forces where it isn’t to be found. And so, I was forced to impress certain of my views on you more explicitly, and to respond, if not to what you asked, at least to what you should have been asking (1705 letter to De Volder, G 2: 275; AG: 181).
His clear exasperation with De Volder notwithstanding, Leibniz does go on to try to explain how bodies still have a place in his monadological metaphysics:
 I do not really do away with body, but reduce it to what it is. For I show that a corporeal mass that is believed to have something besides simple substances is not a substance but a phenomenon resulting from simple substances, which alone have unity and absolute reality. I relegate derivative forces to the phenomena, but I think that it is clear that primitive forces can be nothing other than the internal strivings of simple substances, by which they pass from perception to perception by a certain law of their nature and at the same time agree with one another, representing the same phenomena of the universe in a different manner, something that necessarily arises from a common cause. It is necessary that these simple substances exist everywhere and that they be self-governing (each as far as itself is concerned) since the influence of one on another cannot be understood. Anything more beyond this in things is posited in vain and added without argument (1705 letter to De Volder, PL: 318–319).
These further explanations of Leibniz’s ontological scheme did not lead De Volder to take his ontological framework more seriously.The correspondence thus ended largely in failure. There is no indication that the correspondence led Leibniz to question his idealist ontological framework. It did, however, underscore the difficulties involved in trying to present this theory to people who held views that were far removed from it. It led him to redouble his efforts to find effective pedagogical strategies for presenting the theory of monads to his interlocutors.
Leibniz’s correspondence with De Volder is at the center of recent interpretive debates over his views on fundamental ontology. It contains several of his most famous statements of idealism (texts  and ) and the text that has most frequently been cited as indicating a commitment to the substantiality of organisms in his later works (text ). Commentators have attempted to make sense of this prima facie tension in several different ways. Some people claim that Leibniz’s views on fundamental ontology changed over time. Others suggest that the texts indicate a tension in Leibniz’s thought that he never satisfactorily resolved. Yet others have suggested that text  is, upon closer inspection, actually compatible with an idealist ontology. These idealist readings have been criticized for implausibly reading the theory of monads into texts that are not really in keeping with a commitment to idealism. If we let the texts speak for themselves, it is said, we will see that Leibniz’s supposedly unwavering commitment to idealism is an untenable interpretive dogma.
This sort of criticism is, in my opinion, fundamentally misguided. It presupposes that when Leibniz composed his essays and letters he was always trying to straightforwardly present his views to his interlocutors. As we have seen, this presupposition is not in keeping with Leibniz’s own remarks on how he presents his metaphysics. Leibniz thought that it would be a mistake to straightforwardly present to people the esoteric content of his metaphysics that was far removed from received opinions. He thought the theory of monads was one of the aspects of his metaphysics that was furthest from received opinions. It was far removed from received opinions in two ways. First, it conflicted with the nearly ubiquitous belief that bodies are substances. Second, although people often spoke of the soul, they were not in the habit of properly conceiving of purely intelligible entities that cannot be known through the senses or the imagination. Given these common prejudices, Leibniz thought that if he straightforwardly presented his monadological metaphysics to people they would be likely to misunderstand it, regard it as absurd, and reject it.
Leibniz attempted to counter this problem by composing exoteric texts that were less than fully explicit about his idealist ontology. He used a range of rhetorical strategies to show his interlocutors that simple, immaterial, mind-like substances exist without explicitly drawing (i.e., by selectively omitting) the conclusion that these are the only true substances. In some contexts he retained the language of “corporeal substance” and “composite substance” in characterizing his own accounts of organisms and bodies. Leibniz realized this use of familiar language would lead a number of his interlocutors to take him to be attributing more reality to bodies than they actually possess in his ontology. This minor misunderstanding would enable them to consider many of the virtues of the theory of monads without focusing on the fact that bodies and animals are not substances, strictly speaking. Leibniz thought that if the latter thesis were an initial point of focus his readers would be likely to infer erroneously that bodies are mere illusions—a conclusion they would regard as utterly absurd given their commitment to the substantiality of bodies. Leibniz thought that once his interlocutors were able to appreciate the myriad reasons for postulating monads, and were made aware of the weaknesses of rival theories of substance, they would be better placed to understand that monads are the only true substances, but that organisms and bodies are nevertheless “real” and occupy important places in his monadological metaphysics.
One of the central lessons of this paper is that one cannot adequately address Leibniz’s views on fundamental ontology merely by focusing on the famous snippets of text where Leibniz is thought to make important ontological pronouncements (e.g., texts , , and ). One must take into consideration his remarks on how he presents his philosophy and carefully examine the broader context within which his remarks on fundamental ontology are written. I have used this strategy to examine Leibniz’s remarks on fundamental ontology in the De Volder correspondence. I have highlighted a number of overlooked remarks that strongly suggest that Leibniz was using pedagogical exoteric writing to gradually reveal his monadological metaphysics to De Volder. These additional remarks, in conjunction with Leibniz’s general remarks on exoteric writing, allow one to provide a perfectly natural idealist reading of Leibniz’s purportedly non-idealist remarks in text , and of the correspondence as a whole.
Much more work must be done in order to provide a full defense of an idealist interpretation of Leibniz’s late metaphysics. To do that, careful, contextually-sensitive readings of all of Leibniz’s remarks on fundamental ontology must be provided. I believe that the framework I have developed for understanding how Leibniz uses pedagogical exoteric writing will allow one to make good sense of the full range of Leibniz’s remarks on fundamental ontology. Defending this broader thesis is a task for another day.
Earlier versions of this paper were presented at the Chicago Modern Philosophy Roundtable (Roosevelt University), The Third Annual Finnish-Hungarian Seminar in Early Modern Philosophy (Central European University), the Ninth Annual Conference of the Leibniz Society of North America (Ohio State University), and a workshop on Leibniz and Kant on Substance (Institute for the Humanities, University of Illinois at Chicago). I am grateful to members of those audiences for helpful feedback on the paper, particularly Martha Bolton, Stephen Engstrom, Ursula Goldenbaum, Michael Griffin, Tyler Hanck, David Hilbert, Julia Jorati, Paul Lodge, Saja Parvizian, Alex Silverman, Daniel Sutherland, Donald Rutherford, Georgette Sinkler, Stuart Warner, and Shelley Weinberg. Brandon Look and Alan Nelson provided me with insightful written comments on the paper. I am also thankful for the comments from the anonymous referees for Ergo. I would also like to thank Shirley Whipple for helping me prepare the manuscript for publication.
Abbreviations for Works by Leibniz
- A = (1923-). Sämtliche Shriften und Brief. Akademie-Verlag. Cited by series, volume, and page.
- AG = Roger Ariew and Daneil Garber (Eds. and Trans.) (1989). Philosophical Essays. Hackett. Cited by page.
- DSR = G. H. R. Parkinson (Ed. and Trans.) (1992). De Summa Rerum: Metaphysical Papers, 1675-1676. Yale University Press. Cited by page.
- FC = Foucher de Careil (Ed.) (1975). Lettres et opuscules inedits. Georg Olms. Cited by page.
- G = Carl I. Gerhardt (Ed.) (1875–1890). Die philosophischen Schriften von Gottfried Wilhelm Leibniz (Vols. 1–7). Weidman. Cited by volume and page.
- GM = Carl. I Gerhardt (1849–1863). Mathematische Schriften (Vols. 1–7). Halle. Cited by volume and page.
- L = Leroy E. Loemker (1969). Philosophical Papers and Letters (2nd ed.). Reidel. Cited by page.
- LR = Brandon Look and Donald Rutherford (Eds. and Trans.) (2007). The Leibniz-Des Bosses Correspondence. Yale University Press. Cited by page.
- NE = Peter Remnant and Jonathan Bennett (1981). New Essays on Human Understanding. Cambridge University Press. Cited by page.
- PL = Paul Lodge (Ed. and Trans.) (2013). The Leibniz-De Volder Correspondence With Selections from the Correspondence Between Leibniz and Johann Bernoulli. Yale University Press. Cited by page.
- WF = Roger S. Woolhouse and Richard Francks (Eds. and Trans.) (1997). Leibniz’s ‘New System’ and Associated Contemporary Texts. Oxford. Cited by page.
- Adams, Robert (1994). Leibniz: Determinist, Theist, Idealist. Oxford University Press.
- Adams, Robert (1996). Response to Carriero, Mugnai, and Garber. The Leibniz Review 6, 107–125. https://doi.org/10.5840/leibniz199666
- Arthur, Richard (1998). Infinite Aggregates and Phenomenal Wholes. The Leibniz Review 8, 25–45. https://doi.org/10.5840/leibniz199882
- Baxter, Donald L. M (1995). Corporeal Substances and True Unities. Studia Leibnitiana 27(2), 157–184.
- Broad, C. D (1975). Leibniz: An Introduction. Cambridge University Press.
- Cover, J. A. and John O’Leary-Hawthorne (1999). Substance and Individuation in Leibniz. Cambridge University Press. https://doi.org/10.1017/cbo9780511487149
- Dascal, Marcelo (Ed.) (2008). Leibniz: What Kind of Rationalist? Springer Verlag.
- Fichant, Michel (2003). Leibniz et les machines de la nature. Studia Leibnitiana 35(1), 1–28.
- Garber, Daniel (1985). Leibniz and the Foundations of Physics: The Middle Years. In Kathleen Okruhlik and James Robert Brown (Eds.), The Natural Philosophy of Leibniz (27–130). Dordrecht. https://doi.org/10.1007/978-94-009-5490-8_3
- Goldenbaum, Ursula (2017). Why Leibniz Was No Eclectic. Manuscript in preparation.
- Garber, Daniel (2009). Leibniz: Body, Substance, Monad. Oxford University Press. https://doi.org/10.1093/acprof:oso/9780199566648.001.0001
- Hartz, Glenn (1998). Why Corporeal Substances Keep Popping Up in Leibniz’s Later Philosophy. British Journal for the History of Philosophy 6(2), 193–207. https://doi.org/10.1080/09608788808570990
- Hartz, Glenn (2007). Leibniz’s Final System: Monads, Matter and Animals. Routledge.
- Hartz, Glenn, and Catherine Wilson (2005). Ideas and Animals: The Hard Problem of Leibnizian Metaphysics. Studia Leibnitiana 37(1), 1–19.
- Lodge, Paul (2014). Corporeal Substances as Monadic Composites in Leibniz’s Later Philosophy. In Adrian Nita (Ed.), Between Continuity and Transformation: Leibniz on Substance and Substantial Forms (107–124). Springer.
- Look, Brandon (1999). Leibniz and the ‘vinculum Substantiale.’ Steiner.
- Look, Brandon (2000). Leibniz and the Substance of the Vinculum Substantiale. Journal of the History of Philosophy 38(2), 203–220. https://doi.org/10.1353/hph.2005.0075
- Loptson, Peter (1999). Was Leibniz an Idealist? Philosophy 74(03), 361–385. https://doi.org/10.1017/S0031819199000455
- Loptson, Peter and Richard Arthur (2006). Leibniz’s Body Realism: Two Interpretations. The Leibniz Review 16, 1–42. https://doi.org/10.5840/leibniz2006161
- McDonough, Jeffrey (2013). Leibniz’s Conciliatory Account of Substance. Philosopher’s Imprint 13(6), 1–23.
- Melzer, Arthur (2014). Philosophy between the Lines: The Lost History of Esoteric Writing. University of Chicago Press. https://doi.org/10.7208/chicago/9780226175126.001.0001
- Mercer, Christia (2001). Leibniz’s Metaphysics: It’s Origins and Development. Cambridge University Press. https://doi.org/10.1017/cbo9780511498268
- Nelson, Alan (2007). The Rationalist Impulse. In Alan Nelson (Ed.), A Companion to Rationalism (3–11). Blackwell.
- Nelson, Alan (2013). Philosophical Systems and Their History. In Mogens Laerke, Justin Smith, and Eric Schliesser (Eds.), Philosophy and its History (236–257). Oxford University Press. https://doi.org/10.1093/acprof:oso/9780199857142.003.0012
- Puryear, Stephen (2016). Leibniz on the Nature of Phenomena. In Wenchao Li, et al. (Eds.), Für Unser Glück oder das Glück Anderer: Vortrage des X. Internationalen Leibniz-Kongresses (v. V 169–177). Georg Olms Verlag.
- Riley, Patrick (1996). Leibniz’s Universal Jurisprudence: Justice as Charity of the Wise. Harvard University Press. https://doi.org/10.4159/harvard.9780674418394
- Robinet, André (1986). Architectonique disjonctive, automates systémiques et idéalité transcendantale dans l’oeuvre de G. W. Leibniz. Vrin.
- Russell, Bertrand (1937). A Critical Exposition of the Philosophy of Leibniz (2nd ed.). Allen & Unwin.
- Rutherford, Donald (1995). Leibniz and the Rational Order of Nature. Cambridge University Press. https://doi.org/10.1017/cbo9781139172776
- Rutherford, Donald (2008). Leibniz as Idealist. In Daniel Garber and Steven Nadler (Eds.), Oxford Studies in Early Modern Philosophy (Vol. 4, 141–190).
- Schepers, Heinrich (2008). Leibniz’s Rationalism: A Plea Against Equating Soft and Strong Rationality. In Marcelo Dascal (Ed.), Leibniz: What Kind of Rationalist? (17–36). Springer Verlag. https://doi.org/10.1007/978-1-4020-8668-7_2
- Sleigh, Robert C. Jr (1990). Leibniz and Arnauld: A Commentary on their Correspondence. Yale University Press.
- Whipple, John (2015). Leibniz and the Art of Exoteric Writing. Philosophers’ Imprint 15(35), 1–24.
Defenses of broadly realist readings of Leibniz can be found in Garber (1985; 2009), Broad (1975), Hartz (1988, 2007), Phemister (2005), Robinet (1986), Fichant (2003), Loptson (1999), Loptson and Arthur (2006), and Lodge (2014). This list includes commentators who take Leibniz to have endorsed an ontology of corporeal substances only for a certain period of time, and also those who take Leibniz to endorse an ontology where both corporeal substances and monads are substances, strictly speaking. In this paper I will be bracketing the contentious issue of whether Leibniz was committed to an ontology of corporeal substances in the so-called “middle years.”
Daniel Garber has raised some concerns about the labels of “idealist” and “realist.” He notes, among other things, that Leibniz does not describe his own position using these terms (2009: 385–388). Garber goes so far as to suggest that perhaps the debate between idealist and realist interpretations is ultimately focusing on a “bad question” (2009: 387). I agree that Leibniz would have had reservations with this terminology, particularly insofar as it can seem to suggest that someone who is an idealist does not think that bodies are real. From the perspective of Leibniz’s idealism, as I understand it, he still thinks that bodies and organisms are “real” even though they are not substances. I do think, however, that there is a substantive issue between so-called “idealist” and “realist” interpretations. The key issue is whether or not Leibniz endorses an ontology where the only substances, strictly speaking, are simple, immaterial, mind-like entities. Idealist interpretations answer this question affirmatively. Realist interpretations deny this, claiming instead that Leibniz either (1) endorses an ontology where immaterial simples and organisms are substances, or (2) endorses an ontology where the only true substances are organisms.
For a statement of this kind of criticism see Hartz (2007: 1–2, 13–14). Garber also criticizes people for reading the theory of monads into texts, though he is most concerned with attempts to do this with texts from the 1680s, which I will not be discussing in this paper (2009: xix, 224).
Rutherford has suggested that the esoteric/exoteric distinction is important for understanding Leibniz’s views on substance (1995: 281–282). He takes Leibniz’s monadological metaphysics to be part of the esoteric content of Leibniz’s metaphysics that he thought his interlocutors would be likely to misunderstand and reject. I am in complete agreement with Rutherford on these points, which he uses to provide plausible esoteric/exoteric readings of texts like the Principles of Nature and Grace, where Leibniz initially seems to commit himself to the existence of composite substances. The crucial thing that Rutherford is missing, in my opinion, is an account of the pedagogical function of exoteric texts. Rutherford takes Leibniz to distinguish between “the wise” and everybody else. He only reveals his deepest metaphysics to the wise; in texts that will be read by people who are not wise he does not fully reveal his monadological metaphysics. On this account, interlocutors like De Volder and Des Bosses would clearly belong to the category of the “wise.” On this reading, the esoteric/exoteric distinction would be of no use in explaining Leibniz’s remarks on substance in those correspondences. I will argue, in contrast, that Leibniz uses pedagogical exoteric writing extensively even in his correspondences with figures like De Volder.
For accounts of Leibniz’s purported eclecticism see Mercer (2001: 3–62), Riley (1996: 14), and Dascal (2008: 1). For some important reservations about reading Leibniz as an eclectic thinker see Schepers (2008) and Goldenbaum (2017).
Important discussions of this topic in the secondary literature include Look (1999; 2000), Adams (1994: 299–303), Rutherford (1995: 276–281), Look and Rutherford (2007: lvii–lxxix), and Garber (2009: 358–388).
Lodge sees this text as presenting “the most comprehensive statement of his [Leibniz’s] views on substance that is to be found in the De Volder correspondence” (2013: lxxxvi). Adams describes it as “Leibniz’s fullest statement about the structure of a corporeal substance” (1994: 265).
It is also important to consider this passage in the context of the letter within which it appears. As we will see later, there are intimations elsewhere in the letter that corporeal substances are not substances in the strict sense.
See Melzer (2014: 205–234) for an insightful general account of pedagogical esotericism. Christia Mercer has provided a detailed and insightful interpretation of Leibniz’s rhetorical strategies that emphasizes the pedagogical character of Leibniz’s writing. She summarizes her view as follows: “The Rhetoric of Attraction attempts to engage the sectarian reader by using agreeable philosophical terminology and by extolling the virtues of the reader’s sect while attracting attention to the virtues of other philosophical schools; ultimately the goal is to entice the reader to consider the underlying (and usually unstated) assumptions, which Leibniz considers to be true and which he thinks will eventually lead the reader to philosophical enlightenment and intellectual peace” (2001: 57). I completely agree with Mercer that Leibniz’s rhetorical strategies are subtle, and that he intends to “nudge and not push people in the direction of the truth” (2001: 55). However, I think that Leibniz’s pedagogical rhetorical aims need to be understood within the context of his distinction between the esoteric and the exoteric. Also, while I agree that sometimes Leibniz engages sectarian readers by extolling the virtues of that sect while drawing attention to the virtues of other sects, he uses a number of additional rhetorical devices that do not fit this general model.
Consider, for example, the first public presentation of Leibniz’s philosophical system, the Systeme nouveau de la nature et de la communication des substances. In this text he speaks of “true substantial unities [veritables unités substantielles]” (G 4: 482; AG: 142). He says that “There are only atoms of substance, that is, real unities absolutely destitute of parts, which are the source of actions, the first absolute principles of the composition of things, and, as it were, the final elements in the analysis of substantial things [choses substantielles]” (G 4: 482; AG: 142, emphasis in original). He then says “But when corporeal substances [substances corporelles] are contracted, all their organs together constitute only a physical point relative to us. Thus physical points are indivisible only in appearance; mathematical points are exact, but they are merely modalities. Only metaphysical points or points of substance (constituted by forms or souls) are exact and real, and without them there would be nothing real, since without true unities there would be no multitude” (G 4: 482; AG: 142, emphasis in original). Several of Leibniz’s remarks here seem to suggest that only immaterial simples can be substances given that they are “true unities absolutely destitute of parts.” But Leibniz still speaks of “choses substantielles” and “substances corporelles.” On my reading, although Leibniz does not think that bodies or organisms are substances strictly speaking, he uses this familiar language as part of a strategy for presenting (iv) without fully explicitly affirming (v) so as to minimize the likelihood that his readers will infer (iii).
In characterizing Leibniz’s position, De Volder wrote: “A mass of matter, as long as it is divisible to infinity and there is no empty space, has no unity. As long as it is merely passive and impenetrable and resists motion, it can do nothing unless there is added a motive force, an entelechy, which, together with the mass, will make a corporeal substance, a unity, a monad, since you maintain that it is indivisible” (7 October 1702, PL: 248–249).
Pauline Phemister provides a very different reading of this passage. She thinks that Leibniz only takes item  to be a substance, strictly speaking (2005: 49–51). In other words, Phemister takes Leibniz to endorse an ontology of corporeal substances exclusively. Phemister’s reading is grounded in a broader framework according to which Leibniz tries to make his theory of substance seem closer to a Cartesian conception of immaterial thinking substance than it actually is. This strategy of describing a monad as an immaterial mind-like entity is “nothing more than a device to grab the attention of the French philosophical community” (2005:15). In support of her thesis Phemister cites a 1698 letter to Bernoulli, where he writes “I completely approve of your advice, that among Cartesians and the like, we should abstain from mentioning primary matter and substantial form, and be content with mentioning mass, per se passive, and entelechy or primitive activity, soul, life” (GM 3: 552; AG: 169). While I do not have the space to fully engage Phemister’s reading here, my view is that Leibniz wants to avoid those terms because of their scholastic connotations, but not because he is committed to a non-idealist account of primary matter.
Several commentators have suggested that Leibniz is speaking of substance in a loose or popular sense when he speaks of “corporeal substance” in certain parts of the De Volder correspondence. Noting the tension between Leibniz’s claim that substances must be per se unities and the admission that corporeal substances are not per se unities, Look and Rutherford suggest, “To overcome the appearance of inconsistency, Leibniz must hold that when he speaks of “corporeal substance” he is not using the term “substance” in its strict sense, but rather in an extended sense that abandons the assumption that per se unity is an essential property of substance” (2007: liv). Similarly, Lodge writes that the most plausible way of understanding Leibniz’s view in the De Volder correspondence is that “Corporeal substances are aggregates, but ones that possess a kind of intrinsic unity and activity that warrants the title substance in a loose and popular sense” (2013: ci). The position I am defending is in keeping with these claims. The crucial piece that my interpretation adds is an explanation of why Leibniz would use the term “substance” in a less than strict sense at this point in the De Volder correspondence (and elsewhere). It is an instance of Leibniz using the pedagogical exoteric strategy of familiar language to help De Volder move gradually from his received opinions to a place where he could appreciate (and hopefully endorse) the true monadological metaphysics.
This is noted by Look and Rutherford (2007: lii). They also note that in text  Leibniz does not explicitly claim that the corporeal substance is a per se unity. He only says that the “monad dominating the machine” makes the corporeal substance “one.” In the absence of further elaboration, it is reasonable to read text  as attributing a degree of unity to organisms that falls short of the per se unity that Leibniz thinks all true substances must possess. Lodge has recently challenged this reading of text , noting that there are two other texts written around the same time where Leibniz claims that corporeal substances are per se unities (2014). I disagree with Lodge’s reading of these two additional texts. A careful reading of those texts shows that Leibniz is not using the term “corporeal substance” in these texts in the same way that he uses it in text . In the first text that Lodge cites, Leibniz does say that a corporeal substance is “one per se, and not a mere aggregate of many substances” (G 4: 395; AG: 252). However, immediately prior to this he claims: “Primitive active force, which Aristotle calls first entelechy and one commonly calls the form of a substance, is another natural principle which, together with matter or passive force, completes a corporeal substance” (G 4: 395; AG: 252). These remarks strongly suggest that Leibniz is not using “corporeal substance” to refer to item (5) in text  but rather to item (3), the “monad,” which he describes as being “completed” by “primitive entelechy” and “primary matter/primitive passive power.” If Leibniz is using “corporeal substance” in this text as a synonym for “monad” it is not surprising that he characterizes it as a per se unity. The text thus does not commit him to the position that an animal—the “corporeal substance” in text —is a per se unity. A similar reading can be provided for the other text that Lodge cites (22 March 1703 letter to Jaquelot, G 3: 457). It is plausible to see Leibniz as utilizing something like what Adams has called the “qualified monad conception” of corporeal substance in these two texts. A corporeal substance in this sense “is not a monad plus a body, but a monad as having a body” (1994: 269).