He dwelt primarily on the prin ciple of causality, and quite rightly remarked about that that one could not base its truth (indeed not even the objective validity of the concept of an efficient cause in general) on any insight at all,.e., a priori cognition, and thus that the authority of this law is not constituted in the least by its necessity, but only by its merely general usefulness in the course of experience and a subjective necessity arising therefrom, which he called custom.