Introduction In 1905, Einstein published what came to be known as the special theory of relativity, extending the Galilean-Newtonian principle of relativity for uniform motion from mechanics to all branches of physics. Two years later he was ready to extend the principle to arbitrary motion. He felt strongly that there can only be relative motion, as is evidenced, for instance, by his opening remarks in a series of lectures in Princeton in 1921, published in heavily revised form the following year (Einstein 1922c). A typescript based on a stenographer's notes survives for the first two, nontechnical lectures. On the first page of this presumably verbatim transcript we find Einstein belaboring the issue of the relativity of motion in a way he never would in writing: Whenever we talk about the motion of a body, we always mean by the very concept of motion relative motion… we might as well say “the street moves with respect to the car” as “the car moves with respect to the street.”… These conditions are really quite trivial… we can only conceive of motion as relative motion; as far as the purely geometrical acceleration is concerned, it does not matter from the point of view of which body we talk about it. All this goes without saying and does not need any further discussion. (CPAE 7, Appendix C [p. 1]) Although Einstein insists that these points are trivial, we shall see that they are not even true. What makes his comments all the more remarkable is that by 1921 Einstein had already conceded, however grudgingly, that his general theory of relativity, worked out between 1907 and 1918, does not make all motion relative.