Now the propositions of pure reason, especially when they venture beyond all boundaries of possible experience, admit of no test by experiment with their objects (as in natural science): thus to experiment will be feasible only with concepts and principles that we assume a priori by arranging the latter so that the same objects can be considered from two different sides, on the one side as objects of the senses and the understanding for experience, and on the other side as objects that are merely thought at most for isolated reason striving beyond the bounds of experience.