36. THE DOCTRINE OF THE INTERACTIONIST.--There is no man who does not know that his mind is related to his body as it is not to other material things. We open our eyes, and we see things; we stretch out our hand, and we feel them; our body receives a blow, and we feel pain; we wish to move, and the muscles are set in motion. These things are matters of common experience. We all perceive, in other words, that there is an interaction, in some sense of the term, between mind and body. But it is important to realize that one may be quite well aware of all such facts, and yet may have very vague notions of what one means by body and by mind, and may have no definite theory at all of the sort of relation that obtains between them. The philosopher tries to attain to a clearer conception of these things. His task, be it remembered, is to analyze and explain, not to deny, the experiences which are the common property of mankind. In the present day the two theories of the relation of mind and body that divide the field between them and stand opposed to each other are _interactionism_ and _parallelism_. I have used the word "interaction" a little above in a loose sense to indicate our common experience of the fact that we become conscious of certain changes brought about in our body, and that our purposes realize themselves in action. But every one who accepts this fact is not necessarily an interactionist. The latter is a man who holds a certain more or less definite theory as to what is implied by the fact. Let us take a look at his doctrine. Physical things interact. A billiard ball in motion strikes one which has been at rest; the former loses its motion, the latter begins to roll away. We explain the occurrence by a reference to the laws of mechanics; that is to say, we point out that it is merely an instance of the uniform behavior of matter in motion under such and such circumstances. We distinguish between the state of things at one instant and the state of things at the next, and we call the former _cause_ and the latter _effect_. It should be observed that both cause and effect here belong to the one order, the objective order. They have their place in the external world. Both the balls are material things; their motion, and the space in which they move, are aspects of the external world. If the balls did not exist in the same space, if the motion of the one could not be towards or away from the other, if contact were impossible, we would manifestly have no interaction _in the sense of the word employed above_. As it is, the interaction of physical things is something that we can describe with a good deal of definiteness. Things interact in that they stand in certain physical relations, and undergo changes of relations according to certain laws.