No one, of course, will claim that the mental activity of the scientific discoverer is wholly unlike that of any other class of man; but it leads only to confusion to seek to identify processes so unlike as scientific generalization and artistic production. The artist's purpose is the conveyance of a mood. The author of _Macbeth_ employs every device to impart to the auditor the sense of blood-guiltiness; every lurid scene, every somber phrase, serves to enhance the sentiment. A certain picture by Dürer, a certain poem of Browning's, convey in every detail the feeling of dauntless resolution. Again, a landscape painter, recognizing that his satisfaction in a certain scene depends upon a stretch of blue water with a yellow strand and old-gold foliage, proceeds to rearrange nature for the benefit of the mood he desires to enliven and perpetuate. It is surely a far cry from the attitude of these artists manipulating impressions in order to impart to others an individual mood, to that of the scientific discoverer formulating a law valid for all intellects. In the psychology of the present day there is much that is reminiscent of the biological psychology of Aristotle. From the primitive or nutrient soul which has to do with the vital functions of growth and reproduction, is developed the sentient soul, concerned with movement and sensibility. Finally emerges the intellectual and reasoning soul. These three parts are not mutually exclusive, but the lower foreshadow the higher and are subsumed in it. Aristotle, however, interpreted the lower by the higher and not vice versa. It is no compliment to the scientific discoverer to say that his loftiest intellectual achievement is closely akin to fiction, or is the result of a mere brooding on facts, or is accompanied by emotional excitement, or is the work of blind instinct. It will be found that scientific discovery, while predominantly an intellectual process, varies with the nature of the phenomena of the different sciences and the individual mental differences of the discoverers. As stated at the outset the psychology of scientific discovery must be the subject of prolonged investigation, but some data are already available. One great mathematician, Poincaré, attributes his discoveries to intuition. The essential idea comes with a sense of illumination. It is characterized by suddenness, conciseness, and immediate certainty. It may come unheralded, as he is crossing the street, walking on the cliffs, or stepping into a carriage. There may have intervened a considerable period of time free from conscious effort on the special question involved in the discovery. Poincaré is inclined to account for these sudden solutions of theoretical difficulties on the assumption of long periods of previous unconscious work.