 In Kant’s example from § 29 of the Prolegomena, then, we begin from a mere subjective “empirical rule”: that the perception of an illuminated stone is constantly followed by the perception of heat; and we then convert this “empirical rule” into an objective law according to which the very same relationship is now viewed as “necessary and universally valid”. This transformation is effected by the addition of the a priori concept of causality: “the sun is through its light the cause of heat”. It is in precisely this way, more generally, that the categories or pure concepts of the understanding relate to experience: 