 The difficulty with Newton’s view of absolute space comes, not from the epistemological arguments of relationalism, but from Newton’s own conception of force. If force is defined and measured solely by the power to accelerate a body, then obviously the effects of forces—in short, the causal interactions within a system of bodies—will be independent of the velocity of the system in which they are measured. So the existence of a set of equivalent “inertial frames” is imposed from the start by Newton’s laws. Suppose that we determine for the bodies in a given frame of reference—say, the rest frame of the fixed stars—that all observable accelerations are proportional to forces impressed by bodies within the system, by equal and opposite actions and reactions among those bodies. Then we know that these physical interactions will be the same in any frame of reference that is in uniform rectilinear motion relative to the first one. Therefore no Newtonian experiment will be able to determine the velocity of a body, or system of bodies, relative to absolute space. In other words, there is no way to distinguish absolute space itself from any frame of reference that is in uniform motion relative to it. Newton thought that a coherent account of force and motion requires a background space consisting of “places” that “from infinity to infinity maintain given positions with respect to one another” (1687b, p. 8–9). But the laws of motion enable us to determine an infinity of such spaces, all in uniform rectilinear motion relative to each other. The laws furnish no way of singling out any one as “immovable space.”