In this paper I shall argue for the following two related claims. First, a successor version of the scholastic ‘forma-non-afficit’ theorem functions as the key to Kant’s transcendental idealism. Second, drawing on the ‘natural right’ tradition which Kant sees himself being part of, the relation between our cognitive spontaneity and the legislation of the understanding is one of acknowledgment (rather than creation) of the laws of the understanding. This interpretation allows us to make sense of pure concepts and principles of the understanding as the fundamental laws of nature. They rationally constrain our empirical concepts and judgments, and thus warrant the ‘lawfulness in the connection of appearances.’ (Prol., § 36) The crucial point about Kant’s transcendental hylomorphism is that judgments about anything determinable in sensibility, e.g., sense impressions, do not per se conform to pure concepts and principles. They are rather liable to assessment in light of them.