“The physical meaning of is also well known; namely, any velocity which in any time-system is of magnitude is of the same magnitude in every other time-system. No assumption of the existence of a velocity with this property or of the electromagnetic invariance has entered into the deduction of the kinematical equations of the hyperbolic type.” Note that in this definition Dr. Whitehead has failed to identify with either Maxwell’s constant or any other physical magnitude. Thus is some unspecified magnitude; and we may express Dr. Whitehead’s definition in more familiar language as follows: “The critical or maximum velocity is a velocity of undetermined value which may or may not be demanded by the world-structure. If, however, it should exist, it will be given by the same invariant in every Galilean frame, whatever the value of this invariant may be.” In this definition no reference is made to any physical phenomena or laws, and this is the aspect which appears to constitute its superiority in Whitehead’s opinion. But the trouble is that if physical measurements or the existence of physical laws established empirically by the use of rod, clock, etc., are to be discarded, the definition turns out to be no definition at all. For what sort of definition is one which fails to apprise us of the magnitude of the velocity it proposes to define? The definition of the velocity of sound, for example, is that of a velocity of so many feet a second under specified conditions of observation. A definition which failed to give us this information would be no definition of the velocity of sound. Now it might be contended that the precise magnitude of the invariant velocity is a matter of minor importance, and that its property of invariance (mentioned in the definition) is the fundamental point. We may safely say, however, that no physicist could ever subscribe to this view. Our entire outlook on nature depends essentially on the value of this invariant velocity. If it is finite, and if, in addition, it is equal in value to Maxwell’s constant , hence to the velocity of light in vacuo, we must conceive of a world of space-time in which the laws of nature will be subject to certain rigid conditions. Also, simultaneity, length and duration will be relative. If the invariant velocity is a finite magnitude greater than the value of Maxwell’s constant , we shall still have a world of space-time in the extended sense, but the relativity of velocity will not hold and we should be able to determine our absolute velocity through space. The rigid restrictions placed on the laws of nature would no longer be present, since the laws of electromagnetics would certainly be in conflict with those restrictions. The entire structure of Einstein’s theory would be overthrown.