 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 processes by means of sensation, and then assemble these events and processes together (via necessary connections) by means of the a priori conditions of the possibility of experience—also results in “necessary and universally valid” empirical causal laws of nature (the sun is through its light the cause of heat) governing the events and processes in question.