These distinctions were not unknown to Poincaré, and he wrote: “By the real velocity of the earth I understand, not its absolute velocity, which is meaningless, but its velocity in relation to the ether.” Therefore the possibility of the existence of a velocity discoverable in relation to the ether was not regarded as an absurdity by Poincaré. He said: “Any man who speaks of absolute space uses a word that has no meaning.” It is worth while noticing that in all this the development of Poincaré’s ideas betrays a certain hesitation. Speaking of experiments analogous to those of Michelson, he said: “I know that it will be said that we are not measuring its absolute velocity, but its velocity in relation to the ether. That is scarcely satisfactory. Is it not clear that, if we conceive the principle in this fashion, we can make no deductions whatever from it?” From this it is evident that Poincaré, in spite of himself and all his efforts to avoid it, was disposed to find the distinction between space and ether “scarcely satisfactory.” I must admit that Poincaré’s own argument seems to me not wholly satisfactory, or at least not convincing. “Nature,” says Fresnel, “cares nothing about analytical difficulties.” I imagine that it cares just as little about philosophical or purely physical difficulties. It is hardly an incontestable criterion to suppose that a conception of phenomena is so much nearer to reality the more “satisfactory” it is to us, or the better it is found adapted to the weakness of the human mind. Otherwise we should have to hold, whether we liked or no, that the universe is necessarily adapted to the categories of the mind; that it is constituted with a view to giving us the least possible intellectual trouble. That would be a strange return to anthropocentric finalism and conceit! The fact that vehicles do not pass there, and that pedestrians have to turn back, does not prove that there are no such things as no-thoroughfares in our towns. It is possible, even probable, that the universe also, considered as an object of science, has its no-thoroughfare. Clearly one may reply to me that it is not the universe that is adapted to our mind, but the mind that has become adapted to the universe in the evolutionary course of their relations to each other. The mind needs in its evolution to adapt itself to the universe, in conformity with the principle of minimum action formulated by Fermat: perhaps the most profound principle of the physical, biological, and moral world. In that respect the simplest and most economical ideas are the nearest to reality. Yes, but what proof is there that our mental evolution is complete and perfect, especially when we are dealing with phenomena of which our organism is insensible? * * * * *