Abstract For Kant, the human cognitive faculty has two subfaculties: sensibility and the understanding. Kant’s explanation of the possibility of synthetic a priori cognition turns on the claim that each of these faculties has its own pure forms. Sensibility has space and time; the understanding has the categories. In the Transcendental Aesthetic, Kant argues that space and time are the pure forms of sensibility for human cognizers. But he explicitly leaves open the possibility of there being creatures like us, with both sensibility and understanding, who nevertheless have different pure forms of sensibility. What is the status of the claim that we cannot know whether space and time are the pure forms of sensibility for all discursive cognizers? How does Kant argue for it, and how does it relate to his doctrine of transcendental idealism? This is our topic in this chapter. We suggest that it is not the content of Kant’s argument in the Transcendental Aesthetic which explains his restriction but the method by which he conducts his investigation. And we conclude with some reflections on how this proposal fits with the methodology of the Critical Philosophy more generally.