The first pillar, what we might call Leibniz’s theological irenicism, may itself be thought of as involving three defining commitments. First, like most of his predecessors and contemporaries, Leibniz takes it for granted that, within its own proper domain, philosophy must be an objective science capable of establishing truths with certainty. Thus, for example, he begins his Preliminary Dissertation on the Conformity of Faith with Reason by affirming human reason’s ability to link together truths that “the human mind can attain naturally without being aided by the light of faith” (PD 1/H 73; see also NE IV.xvii.passim; A VI.iv.2341/DA 239). Second, he also upholds, however, a traditional distinction between truths that can be known by unaided human reason and experience, and truths that are “above reason” and can only be known on the basis of revelation (Grua 67–68/DA 425; see also Aquinas 1981, 1–7). Thus, while allowing that there are many truths of mathematics and natural philosophy that we can hope to understand through ordinary reasoning and sensory experience, Leibniz also grants that there are other truths, such as those concerning the nature of the Trinity, the Eucharist, and the Resurrection of the Dead, that we can only hope to understand through divine testimony (NE IV.xvii.23; see also PD 23/H 88; A IV.iv.582/DA 326). Finally, third, Leibniz holds the common — although certainly not uncontested — view that revealed truths must nonetheless be consistent with mundane truths. He thus forcefully insists that “all that which can be refuted in a sound and conclusive manner cannot but be false” and pointedly advises that “neither in divine nor in created matters should contradictory propositions be admitted” (PD 5/H 76; A VI.iv.2341; see also PD 23/H 88/NE IV.xviii/A II.i.171; Grua 62/DA 420; Grua 63/DA 422). These three commitments together represent a core aspect of Leibniz’s overarching conciliatorism insofar as they allow him 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. Although often in the background, this aspect of Leibniz’s overarching conciliatorism comes to the surface explicitly in his ambitious irenic program for reuniting the divided Christian sects of his time. The aim of that program, in brief, is to supply a single, coherent metaphysics that is consistent with the views of all viable, competing Christian sects, with the hope of setting the stage for the mutual toleration of whatever irresolvable differences might remain (A VI.i.494–500/L 109–115; see also A VI.i.501–517). While long overlooked, this aspect of Leibniz’s thinking has recently received much helpful attention, with Maria Rosa Antognazza arguing, for example, that Leibniz’s irenic program “provides a kind of Ariadne’s thread for those who wish to reconstruct the unity underlying Leibniz’s labyrinthine intellectual odyssey” (2008, 90).
