It is the fertility of the Einsteinian system which proves that it is strong and sound. Are the new beings with which it has peopled science—the discoveries predicted by it—legitimate children? The Newtonians say that they are not. But in properly ordered science, as in an ideal State, it is the children that matter, not their legitimacy. At all events the vigorous counter-offensive of M. Painlevé has driven back to their lines the over-zealous apostles of the new gospel, who thought that they had pulverised classic science beyond hope of recovery. Each side now remains in its positions. There is no longer any question of regarding the Newtonian conception of the world as a piece of childlike barbarism. A different conception is now opposed to it—that is all. The war between them is as yet undecided, and may remain for ever undecided, as the weapons with which it might be possible to bring it to an issue are sealed up for ever in the arsenal of metaphysics. * * * * * Whatever may happen, Einstein’s teaching has a power of synthesis and prediction which will inevitably incorporate its majestic system of equations in the science of the future. M. Émile Picard, perpetual secretary of the Academy of Sciences, and one of the luminous and profound thinkers of our time, has asked if it is an advance “to try, as Einstein has done, to reduce physics to geometry.” Without lingering over this question, which may be insoluble, like all speculative questions, we will conclude with the distinguished mathematician that the only things which matter are the agreement of the final formulæ with the facts and the analytic mould in which the theory casts the phenomena. Considered from this angle, Einstein’s theory has the solidity of bronze. Its correctness consists in its explanatory force and in the experimental discoveries predicted by it and at once verified. What changes in theories are the pictures we form of the objects between which science discovers and establishes relations. Sometimes we alter these pictures, but the relations remain true, if they are based upon observed facts. Thanks to this common fund of truth, even the most ephemeral theories do not wholly die. They pass on to each other, like the ancient runners with their torch, the one accessible reality: the laws that express the relations of things. To-day it happens that two theories together clasp the sacred torch. The Einsteinian and the Newtonian vision of the world are two faithful reflections of it: just as the two images, polarised in opposite directions, which Iceland spar shows us in its strange crystal both share the light of the same object.