Now, to one who conceives the mind in a grossly material way, the relation of mind and body can scarcely seem to be a peculiar problem, different from the problem of the relation of one physical thing to another. If my mind consists of atoms disseminated through my body, its presence in the body appears as unequivocal as the presence of a dinner in a man who has just risen from the table. Nor can the interaction of mind and matter present any unusual difficulties, for mind is matter. Atoms may be conceived to approach each other, to clash, to rearrange themselves. Interaction of mind and body is nothing else than an interaction of bodies. One is not forced to give a new meaning to the word. When, however, one begins to think of the mind as immaterial, the case is very different. How shall we conceive an immaterial thing to be related to a material one? Descartes placed the mind in the pineal gland, and in so far he seemed to make its relation to the gland similar to that between two material things. When he tells us that the soul brings it about that the gland bends in different directions, we incline to view the occurrence as very natural--is not the soul in the gland? But, on the other hand, Descartes also taught that the essence of mind is _thought_ and the essence of body is _extension_. He made the two natures so different from each other that men began to ask themselves how the two things could interact at all. The mind wills, said one philosopher, but that volition does not set matter in motion; when the mind wills, God brings about the appropriate change in material things. The mind perceives things, said another, but that is not because they affect it directly; it sees things in God. Ideas and things, said a third, constitute two independent series; no idea can cause a change in things, and no thing can cause a change in ideas. The interactionist is a man who refuses to take any such turn as these philosophers. His doctrine is much nearer to that of Descartes than it is to any of theirs. He uses the one word "interaction" to describe the relation between material things and also the relation between mind and body, nor does he dwell upon the difference between the two. He insists that mind and matter stand in the one causal nexus; that a change in the outside world may be the _cause_ of a perception coming into being in a mind, and that a volition may be the _cause_ of changes in matter. What shall we call the plain man? I think we may call him an interactionist in embryo. The stick in his hand knocks an apple off of the tree; his hand seems to him to be set in motion because he wills it. The relation between his volition and the motion of his hand appears to him to be of much the same sort as that between the motion of the stick and the fall of the apple. In each case he thinks he has to do with the relation of cause and effect.