In short, unless pilots are to be permitted to ram ships on to the rocks and then say that heaven is the only true harbour; unless judges are to be allowed to let murderers loose, and explain afterwards that the murder had done good on the whole; unless soldiers are to be allowed to lose battles and then point out that true glory is to be found in the valley of humiliation; unless cashiers are to rob a bank in order to give it an advertisement; or dentists to torture people to give them a contrast to their comforts; unless we are prepared to let loose all these private fancies against the public and accepted meaning of life or safety or prosperity or pleasure--then it is as plain as Punch's nose that no scientific man must be allowed to meddle with the public definition of madness. We call him in to tell us where it is or when it is. We could not do so, if we had not ourselves settled what it is. As I wish to confine myself in this chapter to the primary point of the plain existence of sanity and insanity, I will not be led along any of the attractive paths that open here. I shall endeavour to deal with them in the next chapter. Here I confine myself to a sort of summary. Suppose a man's throat has been cut, quite swiftly and suddenly, with a table knife, at a small table where we sit. The whole of civil law rests on the supposition that we are witnesses; that we saw it; and if we do not know about it, who does? Now suppose all the witnesses fall into a quarrel about degrees of eyesight. Suppose one says he had brought his reading-glasses instead of his usual glasses; and therefore did not see the man fall across the table and cover it with blood. Suppose another says he could not be certain it was blood, because a slight colour-blindness was hereditary in his family. Suppose a third says he cannot swear to the uplifted knife, because his oculist tells him he is astigmatic, and vertical lines do not affect him as do horizontal lines. Suppose another says that dots have often danced before his eyes in very fantastic combinations, many of which were very like one gentleman cutting another gentleman's throat at dinner. All these things refer to real experiences. There is such a thing as myopia; there is such a thing as colour-blindness; there is such a thing as astigmatism; there is such a thing as shifting shapes swimming before the eyes. But what should we think of a whole dinner party that could give nothing except these highly scientific explanations when found in company with a corpse? I imagine there are only two things we could think: either that they were all drunk, or they were all murderers.