9 6 D O C U M E N T 8 6 O C T O B E R 1 9 2 5 86. To Miguel Masriera Rubio[1] [Berlin,] 7 October 1925 Dear Sir, At last I have found the time to read your thoughtful[2] expositions,[3] which were sent to me by Prof. Weyl many months ago.[4] [I agree with almost all of your considerations. Perhaps they already implicitly contain what I am presenting here.] It surely must be excusable that I did not express myself about Bergson’s remarks.[5] The reason is that no true expert will encounter a problem here. Bergson’s critique: Even though it is proven that M ′, considered from system K, does not receive two light signals simultaneously, it does not follow that the events that constitute these two receptions—considered from system K' or by the observer M '—are not simultaneous as well. Rebuttal: The spacio-temporal concurrence (coincidence) of two point-events is independent of the frame of reference likewise for a noncoincidence. (Without this presupposition, the spatio-temporal arrangement of objects in this world would have no legitimacy whatsoever.) The two signal perceptions of M ' are two point- events. If it is proven that they do not coincide with reference to K, then it follows that they also do not coincide with reference to K'. In short: Bergson forgot that according to relativity theory too, spatio-temporal coincidence is absolute in character. ________________________ A second comment on your statements: If, judging from K', M' is the midpoint of A'B', then the same is valid also with reference to K for a particular time t from K. For the distances A'M ' and M 'B' nec- essarily have the same Lorentz contraction (provided such exists at all) (homoge- neity of space). This is needed to prove that at the K-time of the signal emissions M and M ' coincide. For years now I have not been responding to publications that raise objections to the special theory of relativity. This would be a waste of time which one cannot al- low oneself, considering the brevity of life. Respectfully yours, A. Einstein