Yet the description of these complex facts is only the beginning of psychology. We saw that the real aim is their explanation. III MIND AND BRAIN The central aim of the psychologist must be to explain the mental facts. It is not sufficient to describe the procession of mental experiences in us, we must understand the causes which determine that now this and now that appears and disappears, and appears just in this combination of elements. The astronomer is not satisfied with describing the stars, he wants to explain their movements and to determine which movements are to be expected. The psychologist, like the naturalist, aims towards explanation, and it is this demand which forces him to look from the psychical facts to the physical ones, from the mind to the brain. He is under an illusion if he fancies that he can explain mental facts by themselves. The purposive mind has its connection in itself, the causal psychological mind demands for its connection the body. To understand this necessity is the first step towards understanding the relation of mind and brain. The psychologist's problem of explanation is in one way entirely different from that of the physicist. The physicist finds a world of an unlimited number of atoms which are ultimately conceived as all alike, but each one in a different place, and all the changes in the universe, the movements of the stars, the waves of the ocean, are to be explained by the causal connections of the movements of these atoms. The psychologist, on the other hand, finds an endless manifoldness of elements which are not in space, and which have no space form whatever. My will is neither triangular nor oval; my emotion is neither shorter than five feet nor longer; my memory image of a melody has no thickness and no tallness; my contents of consciousness are as such not in space; their elements cannot pass through any space movements like the atoms of the physicist. Instead of it, the psychical atoms, the sensations, have different qualities, are blue and green, and cold and warm, and sweet and sour, and toothache and headache. The changes which go on in such a system are thus not changes of position and movements, but changes in kind and strength and vividness and fusion; and exactly such changes are the processes which the psychologist wants to explain. He wants to make us understand why this idea grows up and the other fades away, why this impression stands out with clearness as an attended object while the other lacks vividness and disappears, why this volition grows out of that emotion, why this feeling leads to this imaginative thought.