Really good observers are very rare. Napoleon pretended to recall every face he had looked upon once. This has become legendary, but it is not quite so. He confused all the names. One day, he sees a certain face in a deputation and thinks that he recognizes it. It was a scholar who was well known in that day, named Ameilhon. The following dialogue takes place: "Aren't you Ancillon?"--"Yes, sire, Ameilhon."--"Librarian of Sainte-Geneviève?"--"Yes, sire, of the Arsenal."--"Continuator of the History of the Ottoman Empire"--"Yes, sire, of the History of the Low Empire." After which Ameilhon, enchanted with the honor, went off, declaring everywhere most emphatically: "The emperor is amazing. He knows everything." And we, in our turn, might say: men are amazing; they imagine that it is enough to have witnessed an event to be sure of that event! The matter is far more complicated. Certainty is difficult to acquire. Nothing is more difficult than that which is too easy. Nobody would imagine that he could play the violin without having learned how; and if he did, the least attempt would at once extinguish his pretense. But to see? What more simple than that? All one has to do is open one's eyes. "I saw it," is the reply of a witness whose story is contested; "Do you take me for a fellow suffering from hallucination?" Precisely, or else for a purblind person, as the case may be. As a matter of fact, when it comes to seeing, men display two tendencies: they see what they wish to see, what is useful to them, what is agreeable. The second is the tendency toward inhibition; they do not see what they do not wish to see, what is useless to them, or disagreeable. The great rule by which almost everything may be explained, is the rule of utility. Certain artisans were visiting the Universal Exposition. They looked about, walked along, and had seen nothing. Farther on they continued to look about, and this time they stopped; they had caught sight of a machine that could be of use to them in their particular work. We do not see that to which we are indifferent. The image glides by, fades and dies out before having had time to become fixed, and we make no effort to retain it.