One of the principal obstacles in the study of these sciences is thus the way we enter into them, which we can only do by blindly taking the presuppositions for granted and, without being able to form any further concept of them, often not even an exact representation, at best by conjuring up in phantasy a confused picture of them, we right there impress in our memory the determinations of the forces and matters that we have assumed, their hypothetical shapes, their directions and rotations.