When we have to plunge into the depths of the unknown to discover something new, when we have to learn more and better, the end justifies the means. When he reminds us of optics, mechanics, and gravitation, now bound up together in a new sheaf, of the deviation of light by gravity which he foretold against all expectation, of the anomalies of Mercury which he was the first to explain, and of his improvement of the Newtonian law, Einstein has the right to say, with some pride: “There is what I have done.” It is said that the paths by which he attained all these fine results are not devoid of unpleasant false turns and quagmires. Well, there are many ways to Rome and to truth, and some of them are not perfect. The main thing is to get there. And in this case the truth means ancient facts brought into a new harmony, and new facts set forth in prophetic equations and verified in the most surprising manner. If discussion of principles—if theory, which is only the servant of knowledge—shrugs its servile and disloyal shoulders a little over Einstein’s work, at all events experience, the sole source of truth, has justified him. Brilliant formulæ that Einstein had not foreseen are now discovered to explain the anomaly of Mercury and the deviation of light. It is good: but we must not forget that the first of these correct formulæ, that of Einstein, went boldly in advance of the verification. New trenches have been won in the war against the eternal enemy, the unknown. Certainly we have now to organise them and create more direct roads to them. But to-morrow we shall have to advance again, to gain more ground. We shall have, by any theoretical device that we can, to state other new facts, unknown but verifiable facts. That is what Einstein did. If it is a weakness of Einstein’s teaching to deny all objectivity, all privilege, to any system of reference whatever, while utilising such a system for the necessities of calculation, it was at all events a weakness shared by the great Poincaré. To the day of his death he rebelled energetically against the Newtonian conception. The support of such a genius, whom one finds involved in all our modern discoveries, is enough to secure some respect for the Relativist theory. If we have on the one side Newton and his ardent and persuasive apologist, equipped with a fine mathematical genius, Paul Painlevé, we have on the other side Einstein and Henri Poincaré. Even in earlier history we have Aristotle against Epicurus, Copernicus against the Scholastics, at the same barricade. It is an eternal war of ideas, and it may be endless if, as Poincaré believed, the Principle of Relativity is at the bottom only a convention with which experience cannot quarrel because, when we apply it to the entire universe, it is incapable of verification.