natural phenomena, such as the rate of propagation of light, undergo no alteration when we pass from one system of movement of translation to another, as we {51} are constantly doing in the changing velocity of the earth's movement round the sun, is that space and time alter with the velocity. I cannot here give the argument or describe the experiments which have given this result--I am simply taking it as an illustration.[1] It seems to me admirably suited to compare the pragmatist method and the pragmatist attitude with that of scientific realism and of absolute idealism. [1] The Principle of Relativity is mainly the result of the recent mathematical work of H. A. Lorentz, Einstein, and the late Professor Minkowski. A very interesting and not excessively difficult, account of it is contained in _Dernières Pensées_, by the late Henri Poincaré; Paris, Alcan. Here, then, is a question in which the truth of our accepted notions is called in question, and new notions claim to be true. The sole question involved, pragmatism insists, is the truth of conceptions, not the reality of things, and there is but one way of testing the truth of conceptions--and that is by comparing the rival conceptions in respect of the practical consequences that follow from them and adopting those that will work. If the old conceptions of space and time fail to conform to a new need, then what was true before the need was revealed is no longer true, the new conception has become true. By verifying the new conception, we make it true. But, objects the realist, an idea cannot become true; what is now true always was true, and what is no longer true never was true, though we may have worked with the false notion ignorant that it was false. Behind truth there is reality. The earth was spherical even when all mankind believed it flat and found the belief work. To this the pragmatist reply is that reality is only our objectification of truth; it possesses no meaning divorced from human purposes. Had anyone announced that the earth was a sphere {52} when it was generally held to be flat, unless his announcement had some relevance to a defect in the flat earth notion, or a claim to revise that notion, his announcement would have been neither a truth nor a falsehood in any intelligible meaning of the term--he would have been making an irrelevant remark. The notions of space and time that Newton held worked, and were therefore true; if a new need requires us to replace them with other notions, and these other notions will work and are therefore true, they have become true and Newton's notions have become false. If it is still objected that the new notions were also true for Newton, although he was ignorant of them, the need for them not having arisen, the only reply is that truth, or reality, in complete detachment from human purposes, cannot be either affirmed or denied.