Thus, in the example from § 29 of the Prolegomena, Kant begins from a mere “empirical rule” (that heat always follows illumination by the sun) and then proceeds to a “necessary and universally valid” law by adding the a priori concept of cause to this (so far) merely inductive rule. The very same three-stage procedure described by the three Postulates as a whole—in which we begin with the formal a priori conditions of the possibility of experience in general, perceive various actual events and p