it is a heuristic principle for investigating the particular laws of nature; supposing even that we wish to make no use of it for explaining nature itself, — in which we still always speak only of natural purposes, although it apparently exhibits a designed unity of purpose, — i.e. without seeking beyond nature the ground of the possibility of these particular laws. But since we must come in the end to this latter question, it is just as necessary to think for nature a particular kind of causality which does not present itself in it, as the mechanism of natural causes which does. To the receptivity of several forms, different from those of which matter is susceptible by mechanism, must be added a spontaneity of a cause (which therefore cannot be matter), without which no ground can be assigned for those forms. No doubt Reason,