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.

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.