Part II The revolution continued: post-Kantians to rest on suppositions about absolute space that were ruled out by Kant’s own systemof transcendental idealism. Kant was therefore led to see Newton’s absolute space instead as an “Idea” of reason, a conception of an ideal end-point toward which the kinds of judgments that one makes on the basis of a Newtonian systemtend to converge. (That ideal end- point would be the center of mass of the entire universe, something that could never be given in experience. ) However, if the concept of absolute space could not be assumed and could only function instead as a regula- tive ideal in terms of which we investigated nature, then, so Kant argued, we could not go on to do as Newton had done, namely, to use absolute space as the basis for deﬁning the laws of “true” motion (as opposed to “relative” or merely apparent motions, such as the sun “appearing” to move while the earth “appears” to be at rest). Therefore, for Newtonian investigations to be possible in the ﬁrst place, we must have a method for distinguishing true fromapparent motion, which required investigations that rested on a priori presuppositions about the nature of what was movable – which, for Kant, was equivalent to determining the a priori determinations of the empirically constituted conception of matter. This, in turn, led Kant to hold that there must a priori be two different forces at work in matter, those of attraction and repulsion. Attraction is necessary because, in presupposing a center of mass, we need a concept of univer- sal gravitation, of matter as exhibiting essentially a universal attraction for all other matter; in doing that, however, we must also presuppose a countervailing force of repulsion, since if there were only attraction, all matter would condense to one point (just as, if there were only re- pulsion, all matter would scatter into virtual nothingness). Mechanics, Kant concluded, rests on a priori determinations more properly set by transcendental philosophy. Absolute space, like the idea of a common center of mass, is thus, for Kant, an Idea of reason.