It should be observed here that men of genius, at all events, if men of science, often present that species of mania which Wechniakoff[77] and Letourneau[78] have called _monotypic_. Such men occupy themselves throughout their whole lives with one single problem, the first which takes possession of their brains, and which henceforth rules them. Otto Beckmann was occupied during the whole of his life with the pathology of the kidneys; Fresnel with light; Meyer with ants. Here is a new and striking point of resemblance with monomaniacs. On account of this exaggerated and concentrated sensibility, it becomes very difficult to persuade or dissuade either men of genius or the insane. In them the roots of error, as well as those of truth, fix themselves more deeply and multiplexly than in other men, for whom opinion is a habit, an affair of fashion, or of circumstance. Hence the slight utility of moral treatment as applied to the insane; hence also the frequent fallibility of genius. In the same way we can explain why it is that great minds do not seize ideas that the most vulgar intelligence can grasp, while at the same time they discover ideas which would have seemed absurd to others: their greater sensibility is associated with a greater originality of conception. In exalted meditation thought deserts the more simple and easy paths which no longer suit its robust energy. Thus Monge resolved the most difficult problems of a differential calculus, and was embarrassed in seeking an algebraic root of the second degree which a schoolboy might have found. One of Lulli’s friends used to say habitually on his behalf: “Pay no attention to him; he has no common sense: he is all genius.” _Paræsthesia._--To the exhaustion and excessive concentration of sensibility must be attributed all those strange acts showing apparent or intermittent anæsthesia, and analgesia, which are to be found among men of genius as well as among the insane. Socrates presented a photo-paræsthesia which enabled him to gaze at the sun for a considerable time without experiencing any discomfort. The Goncourts, Flaubert, Darwin had a kind of musical daltonism.