 Already in his prize essay of 1764, the Investigation of the Intelligibility of the Fundamental Principles of Natural Theology and Morals, Kant claimed that moral and theological principles were not capable of demonstration, since their terms, unlike mathematical terms, lacked precise definition. Geometrical concepts lend themselves to use in demonstrations because they are constructed and presented to intuition, which is impossible with respect to metaphysical concepts such as the soul. In the Critique of Pure Reason, he says: “If anyone were to pose the question to me: What is the constitution of a thing that thinks? then I do not know the least thing to answer a priori, because the answer ought to be synthetic…But for every synthetic solution, intuition is necessary; but this is entirely left out of so universal a problem” (KRV A 398). The Prolegomena returns to the question how metaphysics can, like natural science and mathematics, employ synthetic judgments when its concepts are not given in experience and are not constructed. Kant’s answer is that metaphysical judgments do not point to objects existing beyond all possible experience, but posit objects needed to “complete” our understanding, that is, to make our thinking systematic and untroubled by gaps and aporias. The soul is not a supersensible object of whose faculties and powers we can acquire knowledge but an idea that makes our practice of ascribing experiences to ourselves intelligible.