Under these circumstances, to what does the example of the two twins finally reduce? Simply to this: The departure of our twin brother and his return to earth constitute two definite events. The duration separating two events being robbed of any definite value by the theory of relativity, there is no cause to be surprised that this duration should manifest different magnitudes to different observers. Again let us put the paradox in the following alternative form: Our twin brother covers a distance of thirty-two light years (when he goes to the star and then returns), and the duration of his trip is forty years; hence his speed along the distance is four-fifths that [Pg 236] of light. How, then, could he ever be misled into believing that he had required only twenty years to cover this total distance? Would not this belief suggest to him that his speed had been greater than that of light, and would not this fact conflict not only with relativity but also with the assertion which he would voluntarily offer, that his speed with respect to us was only four-fifths that of light? But here, again, the criticism is occasioned by another erroneous obsession, that of absolute distance. The critic does not grasp the fact that the distance from earth to star, just like any other distance, is indeterminate and has no meaning in itself. While it is true that, so far as we are concerned, this distance is sixteen light years, yet in the opinion of the traveller it is only eight light years; and his opinion expresses exactly the same measure of reality as does our own. We can only repeat once more that absolute duration and distance are incompatible with Einstein’s premises; and if we insist upon preserving our classical obsessions, a study of Einstein’s theory will be little better than a nightmare. Let us now pass in review a totally different order of criticism. It is contended, for example, that the principle of relativity permits us to interpret any problem of relative motion between two bodies in two different ways: by considering either one body or the other as being at rest. Then, according to the critic, since in the problem of the two twins it is the twin’s motion that is the cause of the slowing down of his time, all we need do is to consider him at rest and his brother in motion, in order to reach the ridiculous conclusion that either twin is younger than his brother. This argument is radically incorrect for two separate reasons. In the first place, the special principle of relativity does not state that, in the general case, when two bodies are in relative motion we can consider either one of them as being at rest. The special principle emphasises the fact that a reversibility of this kind is possible only when the motions of both bodies are Galilean.