Kant presents three principles of modality in the Critique of Pure Reason. Historically, commentators have mostly disregarded them; a few have rejected them outright. In recent years, however, a consensus has begun to develop around the idea that the role of these principles is to rule out certain metaphysical doctrines. I argue that this understates their importance. Rather, the principles of modality are essential conditions of the possibility of experience. I conclude by examining the question of their truth, which, I (...) argue, can only be grasped by considering them as postulates—i.e., as practical propositions of a certain kind. (shrink)