hat 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.

The opponent of the interactionist insists, however, that the plain man
is satisfied with this view of the matter only because he has not
completely stripped off the tendency to conceive the mind as a material
thing. And he accuses the interactionist of having fallen a prey to
the same weakness.

Certainly, it is not difficult to show that the interactionists write
as though the mind were material, and could be somewhere in space. The
late Dr. McCosh fairly represents the thought of many, and he was
capable of expressing himse