In the first Critique, Kant claims to refute Moses Mendelssohn’s argument for the immortality of the soul. But some commentators, following Bennett (1974), have identified an apparent problem in the exchange: Mendelssohn appears to have overlooked the possibility that the “leap” between existence and non-existence might be a boundary or limit point in a continuous series, and Kant appears not to have exploited the lacuna, but to have instead offered an irrelevant criticism. Here, we argue that even if these commentators (...) are correct, an argument against the leap-as-limit possibility is implicit in claims that Mendelssohn accepts. Moreover, Kant’s criticism of Mendelssohn adapts naturally into a response to this argument, though Mendelssohn endorses further claims which enable him to address this Kantian response. To illustrate the philosophical issues in play, we conclude by noting the affinity between the Mendelssohnian argument we develop and several prominent arguments in contemporary metaphysics: David Lewis’s argument from vagueness for unrestricted composition, Ted Sider’s argument from vagueness for perdurantism, and Peter Unger’s argument from the problem of the many for substance dualism. In short, we argue that the philosophical issues involved in the Mendelssohn-Kant exchange are much richer than previous commentators have believed, and that there is a Mendelssohnian argument for the immortality of the soul (or anyway, the permanence of simples) that does not suffer from any obvious flaw. (shrink)