If we will bear in mind what was said in the closing section of the last chapter, we can help him over his difficulty.  That mind and body are related there can be no doubt.  But should we use the word “in” to express this relation?

The body is a certain group of phenomena in the objective order; that is, it is a part of the external world.  The mind consists of experiences in the subjective order.  We have seen that no mental phenomenon can occupy space—­real space, the space of the external world—­and that it cannot even have a position in space (section 34).  As mental, it is excluded from the objective order altogether.  The mind is not, then, strictly speaking, in the body, although it is related to it.  It remains, of course, to ask ourselves how we ought to conceive the relation.  This we shall do later in the present chapter.

But, it may be said, it would sound odd to deny that the mind is in the body.  Does not every one use the expression?  What can we substitute for it?  I answer:  If it is convenient to use the expression let us continue to do so.  Men must talk so as to be understood.  But let us not perpetuate error, and, as occasion demands it, let us make clear to ourselves and to others what we have a right to understand by this in when we use it.

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.