It is commonly claimed that Kant's critical philosophy aims to limit reason's speculative use and its metaphysical pretensions. This paper argues that such claims should be amended in light of a technical distinction between negative limits and positive boundaries that Kant held throughout his career. Kant's only extended discussion of this distinction appears in §§57–60 of the Prolegomena, a division entitled “On pure reason's boundary‐determination”. I examine these sections in detail in order to elucidate the account of the limits and (...) boundaries of the activity of different cognitive faculties in the Critique of Pure Reason. I argue that once we understand a key ambition of the Critique to be the positive determination of the boundary between objects of experience and things in themselves, we gain insight into the essential contribution that Kant's theory of the positive utility of ideas of reason makes to the critical philosophy. (shrink)