In theory even crude observations should have led us to a decision on this score, for crude observations differ from those of the physicist only in the matter of precision. Theoretically, therefore, prior to Einstein we should have known of the incorrectness of the Euclidean composition of velocities and hence known of space-time. The reason for our failure to make the momentous discovery is easily understood when we realise that owing to the very great magnitude of the invariant finite velocity , the Einsteinian rule can scarcely be differentiated from the classical rule unless very great velocities are involved. From this it is seen that the reason why the effects anticipated by Einstein’s theory are so difficult to detect is because of the very high value of the invariant velocity . Had this finite invariant velocity happened to be small, say a mile a second, the classical conception of a world of separate space and time would have been abandoned years ago; for the curious effects predicted by Einstein would have become perceptible without effort. Again, were we to conceive of this invariant velocity as becoming greater and greater, the distinguishing features of the Einsteinian universe would gradually fade away, until finally, if the invariant velocity became infinite, space-time would give way to the separate space and time of classical science, and all the peculiarities of Einstein’s universe would vanish. [Pg 206] A direct study of the Lorentz-Einstein transformations would lead to the same conclusions; for if in these transformations we make infinite, we find that they degenerate into the classical Galilean ones. And here a rather delicate point must be noted. The constant —invariant velocity of the universe—has been identified in Einstein’s theory with the velocity of light in vacuo. This identification was inevitable, as we explained when discussing the equations of electromagnetics. But while it is correct to state that Einstein’s theory would degenerate into classical science were , the invariant velocity of the universe, infinite instead of finite, it would be wrong to infer therefrom that were the velocity of light infinite we should be faced with the world such as classical science thought it to be. As we know, in classical science the velocity of light was not infinite; it was only the invariant velocity that was infinite; and these two velocities had nothing in common.