If a Galilean frame moves away from us with a speed and if, with respect to this frame and in the direction of its motion, a bullet speeds with a velocity , the resultant speed of the bullet might be only provided our methods of measuring a velocity, and those of the observer in the Galilean frame, were identical; that is, provided our measurements of space and time were identical. This was always tacitly assumed to be the case by classical science; but we know that in Einstein’s theory the identity of our standards of measurement no longer holds, since such a state of affairs would be incompatible with the invariance of the velocity of light. We may therefore assimilate the conditions surrounding the various space and time measurements in Einstein’s theory to those which would endure in classical science if by reason of some law of nature it were impossible for the observer in the train to fire a bullet in the precise direction of the train’s motion. In other words, in Einstein’s theory, velocities lying in the same direction add up like algebraical numbers, provided the velocities we propose to add and subtract are computed under exactly the same conditions, that is, by the same Galilean observer. It is when one observer computes one velocity and a second observer, in relative motion, computes the second velocity that the classical law of addition is at fault. Very similar to this problem of the addition of velocities is that of simultaneity. Thus, Einstein’s theory implies that if two events and occur simultaneously for one observer, John, and if in the opinion of another observer, Peter, in motion with respect to John, the event and another event occur simultaneously, it is not correct to assume that and will be simultaneous in the opinion of either of the two observers. Here the critic assumes, without further ado, that the symbolic expression of these relationships would be as follows: ; ; . Therefrom he infers that the theory of relativity is contrary to logic and is a manifest absurdity. And yet, if we examine these statements carefully, we shall have no difficulty in seeing that the error in logical reasoning lies not with Einstein but with the critic himself. In order to dispel any doubt on this score, let us ask the critic what he intends to convey by the symbol =. Suppose, for example, that we are considering two objects, and , of the same size. Are we entitled to write ? The answer should be: Yes, if the symbol = stands for “is of the same size as”; or No, if it stands for most other types of identity. In order to avoid confusion let us then represent the symbol by ( standing for size) and let us write .