even though it is reason, it must nevertheless exhibit an empirical character, because every cause presupposes a rule according to which certain appearances follow as effects, and every rule requires a uniformity in its effects, grounding the concept of a cause (as a faculty), which, insofar as it must come to light from mere appearances, we could call the empirical character, which is constant, while its effects appear in alterable shapes, according to the differences among the conditions that accompany and in part limit it.