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 himself as follows;[1] “It may be difficult to ascertain the exact point or surface at which the mind and body come together and influence each other, in particular, how far into the body (Descartes without proof thought it to be in the pineal gland), but it is certain, that when they do meet mind knows body as having its essential properties of extension and resisting energy.”

How can an immaterial thing be located at some point or surface within the body?  How can a material thing and an immaterial thing “come together” at a point or surface?  And if they cannot come together, what have we in mind when we say they interact?

The parallelist, for it is he who opposes interactionism, insists that we must not forget that mental phenomena do not belong to the same order as physical phenomena.  He points out that, when we make the word “interaction” cover the relations of mental phenomena to physical phenomena as well as the relations of the latter to each other, we are assimilating heedlessly facts of two different kinds and are obliterating an important distinction.  He makes the same objection to calling the relations between mental phenomena and physical phenomena causal.  If the relation of a volition to the movement of the arm is not the same as that of a physical cause to its physical effect, why, he argues, do you disguise the difference by calling them by the same name?