In this formula, s has most definitely the meaning of a ‘thing’, for it represents measured spatial distance. Of the two factors on the other side of the second equation, one must needs have the same quality as s: this is c. Thus for the other factor, t, there remains the property of a pure number. We are, therefore, under an illusion if we assume the factor c to represent anything of what velocity implies in outer cosmic reality. The truth is that c represents a spatial distance just as s does, with the difference only that it is a certain unit-distance. Just as little does real time enter into this formula – nor does it into any other formula of mathematical physics. ‘Time’, in physics, is always a pure number without any cosmic quality. Indeed, how could it be otherwise for a purely kinematic world-observation?