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 archaeological 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.

When a system explains the facts, we hold it as proved just to the extent to which it explains them.  This accordance of our thought with the nature of things is the mark of what we call truth.

If you grant me these premises, my demonstration is completed, and it only remains for me to draw my conclusions.