All the logic of discoveries is laid down in these lines; and these lines are a testimony rendered by one of the most competent of witnesses. You see in them the conditions of a good hypothesis: Kepler has long studied the phenomena of which he wishes to find the law; he has studied them by himself, and by means of the discoveries of his predecessor Brahe. The law has presented itself to his mind at a given moment, on the eighth of March, 1618. But he does not yet know whether it is a true light, or a deceptive gleam. He seeks the confirmation of his hypothesis; he does not find it, because he makes a mistake, and he rejects his idea as useless. The idea returns; a new course of labor confirms it; and so the hypothesis becomes a law, a certain proposition.
Such is the regular march of thought. An hypothesis has no right to be brought forward until it has passed into the condition of a law, by being duly confirmed. There are minds, however, endowed with a sort of divination, which feel as by instinct the truth of a discovery, even before it has been confirmed. It is told of Copernicus, that having discovered, or re-discovered, the true system of planetary motion, he encountered an opponent who said to him: "If your system were true, Venus would have phases like the moon; now she has none, and therefore your system is false. What have you to reply?"—"I have no reply to make," said Copernicus, (the objection was a serious one in fact); "but God will grant that the answer shall be found."[165] Galileo appeared, and by means of the telescope it was ascertained that Venus has phases like the moon;—the confidence of Copernicus was justified. The scientific career of M. Ampère, the illustrious natural philosopher, supplies an analogous fact. Trusting, like Copernicus, to a kind of intuition of truth, he read one day to the Academy of sciences the complete description of an experiment which he had never made. He made it subsequently, and the result answered completely to his anticipations. Genius is here raised to the second power, since it possesses at once the gift of discovery and the just presentiment of its confirmation; but these are exceptional cases, and in general we must say, with Mithridates, that—
. . . . To be approved as true
Such projects must be proved, and carried through.[166]
We would encourage no one to attempt adventures so perilous, but would call to mind in a great example what is the regular march of science. Newton, after he had discovered the law which regulates the motions of the heavens, sought the confirmation of it in an immense series of calculations. A true ascetic of science, he imposed on himself a regimen as severe as that of a Trappist monk, in order that his life might be wholly concentrated upon the operations of the understanding; and it was not until after fifteen months of persistent labor that he exclaimed: "I have discovered it! My calculations have really encountered the march of the stars. Glory to God! who has permitted us to catch a glimpse of the skirts of His ways!" And astronomy, placed upon a wider and firmer basis, went forward with new energy.
It is thus that the human mind acquires knowledge. How then does hypothesis come to be made light of? How can it be seriously said that we have excluded hypothesis from the sphere of science, whereas the moment the faculty of supposing should cease to be in exercise, the march of science would be arrested; since, except a small number of principles the evidence of which is immediate, all the truths we possess are only suppositions confirmed by experiment? The reason is here: Our mind forms a thousand different suppositions at its own will and fancy; and it shrinks from that studious toil which alone puts it in a position to make fruitful suppositions. We are for ever tempted to be guessing, instead of setting ourselves, by patient observations, on the road to real discoveries. It is therefore with good reason that theories hastily built up have been condemned, and Lord Chancellor Bacon was right in thinking that the human mind requires lead to be attached to it, and not wings. Hence the inference has been drawn that the simplest plan would be to cut the wings of thought, without reflecting that thenceforward it would continue motionless. Because some had abused hypothesis, others must conclude that we could do without it altogether.
Trivial and premature suppositions have therefore discredited hypothesis, by encumbering science with a crowd of vain imaginations; but this encumbrance would have been of small importance but for the obstinacy with which false theories have too often been maintained against the evidence of facts. If Ampère had found his experiment fail, and had still continued to maintain his statements, he would not have given proof of a happy audacity, but of a ridiculous obstinacy. Genius itself makes mistakes, and experience alone distinguishes real laws from mere freaks of our thought. We have maintained the rights of reason in the spontaneous exercise of the faculty of discovery; but let us beware how we ignore the rights of experience. It alone prepares discoveries; it alone can confirm them. A system, however well put together, is convicted of error by the least fact which really contradicts it. A Greek philosopher was demonstrating by specious arguments that motion is impossible. Diogenes was one of his auditory, and he got up and began to walk: the answer was conclusive. You remember, if you have read Walter Scott, the learned demonstration of the antiquary who is settling the date of a Roman or Celtic ruin, I forget which; and the intervention of the beggar, who has no archæological system, but who has seen the edifice in question both built and fall to decay. Reason as much as you like; if your reasonings do not accord with facts, you will have woven spider's webs, of admirable fineness perhaps, but wanting in solidity.
It is time to sum up these lengthened considerations. Science does not originate solely from experiment, nor does it proceed solely from reason; it results from the meeting together of experience and reason. Experience prepares the discovery, genius makes it, experience confirms it. What distinguishes the sciences is not the process of invention, which is everywhere the same; but the process of control over supposed truths. A mathematical discovery is confirmed by pure reasoning. A physical discovery is confirmed by sensible observation joined with calculation. A discovery in the order of morals is confirmed by observation of the facts of consciousness. Therefore it is that between the physical and moral sciences there exists a broad line of demarcation. Moral facts have not less certainty than physical phenomena; but moral facts falling under the influence of liberty, all men cannot perceive them equally under all conditions. An optical experiment presents itself to the eyes, and all the spectators see it alike, if at least they have one and the same visual organization; but a case of moral experience has a personal character, and is only communicated to another person on condition that he puts faith in the testimony of his fellow. In this order of things a man can observe directly only what he concurs in producing. With this reservation, we may say that the control of moral truths is made by experience like that of physical truths. In all departments of knowledge, a thought may be held as true when it accounts for facts.
And so, Gentlemen, we conclude that every scientific truth is, in its origin, a supposition of the mind, the result of which is to produce the meeting together of experience and reason, and so to permit the rational reconstruction of the facts.
Every system is shown to be at fault by facts, if facts contradict it.