 Evidently Newton was aware that “absolute space” was not a reference frame in any practical sense. He emphasized that “the parts of space cannot be seen,” and that no observable bodies can be known to be at rest. Hence there is no way to determine motion with respect to space by direct observation; it must be known by its “properties, causes, and effects” (1687b, p. 7–8). The question arises, then, what properties, causes, or effects indicate a body’s change of position in absolute space? It is conceivable, for example, that there might be some physical correlate for velocity, in the sense that a body might have some observable physical state that depends on its velocity. It follows, in that case, that a body would be in a distinct physical state when it is at rest in space. If a body could be known to be in that state, therefore, it would (in principle) provide a physical marker for a truly resting frame of reference. On Leibniz’s conception of force, for instance, a given force is required to generate or to maintain a given velocity. For objects “passively” resist motion, but maintain their states of motion only by “active” force—so that, on dynamical grounds, “every body truly does have a certain amount of motion, or, if you will, force” (Leibniz 1694, p. 184; see also 1716, p. 404). It would follow from this that there must be, in principle, a distinguished frame of reference in which the velocities of bodies correspond to their true velocities, i.e. to the amounts of moving force that they truly possess. It would also follow that, with respect to any frame that is in motion relative to this one, bodies will not have their true velocities. In short, such a conception of force, if it could be applied physically, would give a precise physical application of Newton’s conception of absolute space, by providing a physical correlate for change of absolute place. 