', 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."
Before considering these transformations in detail we shall make a few general remarks about space and time. In the pre-relativity physics space and time were separate entities. Specifications of time were independent of the choice of the space of reference. The Newtonian mechanics was relative with respect to the space of reference, so that, e.g. the statement that two non-simultaneous events happened at the same place had no objective meaning (that is, independent of the space of reference). But this relativity had no role in building up the theory. One spoke of points of space, as of instants of time, as if they were absolute realities. It was not observed that the true element of the space-time specification was the event, specified by the four numbers