In this paper I discuss Poincar?'s solution to the following problem: the principle of relativity is not valid for rotations and we thus can claim for absolute rotation. The principle of relativity was experimentally not valid for uniform rotations, and therefore it lost of its complete validity. Logical conventionalism (the philosophical principle of relativity) also lost of its complete validity. Therefore, since the principle of relativity was not a priori completely valid, we could disclose absolute motions with respect to absolute space, or speak of a reality independent of the observer. Poincar? could not accept this. He therefore postulated the ether as a material body in absolute rest. By doing so he felt that the principle of relativity regained its complete validity, because by no experimental means could we disclose absolute space (1908a, p. 567): ?it is impossible to escape this impression that the principle of relativity is a general law of nature, that one can never by any imaginable means get evidence of any but relative velocities, and by this I mean not merely the velocities of bodies in relation to the ether, but the velocities of bodies in relation to other bodies?.
