It was absolutely necessary to find a way out of this inexplicable contradiction, to end this paradoxical mockery which the facts seemed to oppose to the most rigorous results of calculation. This the men of science succeeded in doing. How? By the method which is generally used in such circumstances—by means of supplementary hypotheses. Hypotheses in science are a kind of soft cement which hardens rapidly in the open air, thus enabling us to join together the separate blocks of the structure, and to fill up the breaches made in the wall by projectiles, with artificial stuff which the superficial observer presently mistakes for stone. It is because hypotheses are something like that in science that the best scientific theories are those which include least hypotheses. But I am wrong in using the plural in this connection. In the end it was found that one single hypothesis conveniently explained the negative result of the Michelson experiment. That is, by the way, a rare and remarkable experience. Hypotheses usually spring up like mushrooms in every dark corner of science. You get a score of them to explain the slightest obscurity. This single hypothesis, which seemed to be capable of extricating physicists from the dilemma into which Michelson had put them, was first advanced by the distinguished Irish mathematician Fitzgerald, then taken up and developed by the celebrated Dutch physicist Lorentz, the Poincaré of Holland, one of the most brilliant thinkers of our time. Einstein would no more have attained fame without him than Kepler would without Copernicus and Tycho Brahe. Let us now see what this Fitzgerald-Lorentz hypothesis, as strange as it is simple, really is. But we must first glance at a preliminary matter of some importance. A number of able men have declared—after the issue, let it be said—that the result of the Michelson experiment could only be negative _a priori_. In point of fact, they argue (more or less), the Classic Principle of Relativity, the principle known to Galileo and Newton, implies that it is impossible for an observer who shares the motion of a vehicle to detect the motion of that vehicle by any facts he observes while he is in it. Thus, when two ships or two trains pass each other,[4] it is impossible for the passengers to say which of the two is moving, or moving the more rapidly. All that they can perceive is the relative speed of the trains or ships. [4] It is assumed that the ship is not rolling or pitching, and that there is no vibration in the train. The men of science to whom I have referred say that, if Michelson’s experiment had had a positive result, it would have given us the absolute velocity of the earth in space. This result would have been contrary to the Principle of Relativity of classical philosophy and mechanics, which is a self-evident truth. Therefore the result could only be negative.