In this paper, I examine Kant's famous objection to the ontological argument: existence is not a determination. Previous commentators have not adequately explained what this claim means, how it undermines the ontological argument, or how Kant argues for it. I argue that the claim that existence is not a determination means that it is not possible for there to be non-existent objects; necessarily, there are only existent objects. I argue further that Kant's target is not merely ontological arguments as such (...) but the larger ‘ontotheist’ metaphysics they presuppose: the view that God necessarily exists in virtue of his essence being contained in, or logically entailed by, his essence. I show that the ontotheist explanation of divine necessity requires the assumption that existence is a determination, and I show that Descartes and Leibniz are implicitly committed to this in their published versions of the ontological argument. I consider the philosophical motivations for the claim that existence is a determination and then I examine Kant's arguments in the Critique of Pure Reason against it. (shrink)