Is Leibniz's Principle of the Identity of Indiscernibles Necessary or Contingent?
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Abstract
The Principle of the Identity of Indiscernibles (PII)—the principle that no two numerically distinct things are perfectly similar—features prominently in Leibniz’s metaphysics. Despite its centrality to his philosophical system, it is surprisingly difficult to determine what modal status Leibniz ascribes to the PII. On many occasions Leibniz appears to endorse the necessity of the PII. There are a number of passages,however (especially in his correspondence with Clarke), where Leibniz seems to imply that numerically distinct indiscernibles are possible, which suggests that he subscribes to a merely contingent version of the PII. In this paper I attempt to resolve this apparent inconsistency. I argue that Leibniz consistently takes the PII to be necessary and that this view of his shines through even in his correspondence with Clarke. I also show that competing interpretations, on which Leibniz’s PII is contingent, misread a number of crucial passages from this correspondence.
