The theory of relativity is often criticized for giving, without justification, a central theoretical role to the propagation of light, in that it founds the concept of time upon the law of propagation of light. The situation, however, is somewhat as follows. In order to give physical significance to the concept of time, processes of some kind are required which enable relations to be established between different places. It is immaterial what kind of processes one chooses for such a definition of time. It is advantageous, however, for the theory, to choose only those [Pg 29] processes concerning which we know something certain. This holds for the propagation of light in vacuo in a higher degree than for any other process which could be considered, thanks to the investigations of Maxwell and H. A. Lorentz. From all of these considerations, space and time data have a physically real, and not a mere fictitious, significance; in particular this holds for all the relations in which co-ordinates and time enter, e.g. the relations (21). There is, therefore, sense in asking whether those equations are true or not, as well as in asking what the true equations of transformation are by which we pass from one inertial system to another, ', moving relatively to it. It may be shown that this is uniquely settled by means of the principle of the constancy of the velocity of light and the principle of special relativity. To this end we think of space and time physically defined with respect to two inertial systems, and ', in the way that has been shown. Further, let a ray of light pass from one point to another point of through a vacuum. If is the measured distance between the two points, then the propagation of light must satisfy the equation If we square this equation, and express by the differences of the co-ordinates, , in place of this equation we can write This equation formulates the principle of the constancy of the velocity of light relatively to . It must hold whatever may be the motion of the source which emits the ray of light. [Pg 30] The same propagation of light may also be considered relatively to ', in which case also the principle of the constancy of the velocity of light must be satisfied. Therefore, with respect to ', we have the equation Equations (22a) and (22) must be mutually consistent with each other with respect to the transformation which transforms from to '. A transformation which effects this we shall call a "Lorentz transformation."