ion. 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.

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 t