 But, so completed, Thomson’s definition makes the essential point about the relation between Newton’s laws of motion and the inertial frames: that the laws assert the existence of at least one inertial frame. If one inertial frame is posited, in which accelerations properly correspond to Newtonian forces, then any other inertial frame is in uniform motion with respect to the first; the forces, masses, and accelerations measured in one will have the same measures in any other. Any one may be arbitrarily designated as an all-encompassing “immobile space” in which all others are moving uniformly. Thus the original question, “relative to what frame of reference do the laws of motion hold?” is revealed to be wrongly posed. For the laws of motion essentially determine a class of reference frames, and (in principle) a procedure for constructing them. For the same reason, a skeptical question that is still commonly asked about the laws of motion—why is it that the laws are true only relative to a certain choice of reference frame?—is also wrongly posed. If Newton’s laws are true, then we can construct an inertial frame; their truth doesn’t depend on our ability to construct such a frame in advance. Mach expressed the situation particularly clearly: