In the sciences dealing with facts, all the best-established experimental truths have passed through a conjectural stage. History permits no doubt on this point. What makes it appear otherwise is the fact that for centuries there has gradually come to be formed a body of solid belief, making a whole, stored away in classic treatises from which we learn from childhood, and in which they seem to be arranged of themselves. We are not told of the series of checks and failures through which[113] they have passed. Innumerable are the inventions that remained for a long time in a state of conjecture, matters of pure imagination, because various circumstances did not permit them to take shape, to be demonstrated and verified. Thus, in the thirteenth century, Roger Bacon had a very clear idea of a construction on rails similar to our railroads; of optical instruments that would permit, as does the telescope, to see very far, and to discover the invisible. It is even claimed that he must have foreseen the phenomena of interferences, the demonstration of which had to be awaited ten centuries. On the other hand, there are guesses that have met success without much delay, but in which the imaginative phase--that of the invention preceding all demonstration--is easy to locate. We know that Tycho-Brahe, lacking inventive genius but rich in capacity for exact observation, met Kepler, an adventurous spirit: together, the two made a complete scientist. We have seen how Kepler, guided by a preconceived notion of the "harmony of the spheres," after many trials and corrections, ended by discovering his laws. Copernicus recognized expressly that his theory was suggested to him by an hypothesis of Pythagoras--that of a revolution of the earth about a central fire, assumed to be in a fixed position. Newton imagined his hypothesis of gravitation from the year 1666 on, then abandoned it, the result of his calculations disagreeing with observation; finally he took it up again after a lapse of a few years, having obtained from Paris the new measure of the terrestrial meridian that permitted him to prove his guess. In relating his discoveries, Lavoisier is lavish in expressions that leave no doubt as to their originally conjectural character. "He _suspects_ that the air of the atmosphere is not a simple thing, but is composed of two very different substances." "He _presumes_ that the permanent alkalies (potash, soda) and the earths (lime, magnesia) should not be considered simple substances." And he adds: "What I present here is at the most no more than a mere _conjecture_." We have mentioned above the case of Darwin. Besides, the history of scientific discoveries is full of facts of this sort.