It is very important to remark that in all this no new meaning has been given to the word “concomitance.”  The plain man remarks that sensations and their changes must be referred to the body.  With the body disposed in a certain way, he has sensations of a certain kind; with changes in the body, the sensations change.  He does not perceive the sensations to be in the body.  As I recede from a house I have a whole series of visual experiences differing from each other and ending in a faint speck which bears little resemblance to the experience with which I started.  I have had, as we say, a series of sensations, or groups of such.  Did any single group, did the experience which I had at any single moment, seem to me to be in my body?  Surely not.  Its relation to my body is other than that.

And when the man of science, instead of referring sensations vaguely to the body, refers them to the brain, the reference is of precisely the same nature.  From our common experience of the relation of the physical and the mental he starts out.  He has no other ground on which to stand.  He can only mark the reference with greater exactitude.

I have been speaking of the relation of sensations to the brain.  It is scarcely necessary for me to show that all other mental phenomena must be referred to the brain as well, and that the reference must be of the same nature.  The considerations which lead us to refer ideas to the brain are set forth in our physiologies and psychologies.  The effects of cerebral disease, injuries to the brain, etc., are too well known to need mention; and it is palpably as absurd to put ideas in the brain as it is to put sensations there.

Now, the parallelist, if he be a wise man, will not attempt to explain the reference of mental phenomena to the brain—­to explain the relation between mind and matter.  The relation appears to be unique.  Certainly it is not identical with the relation between two material things.  We explain things, in the common acceptation of the word, when we show that a case under consideration is an exemplification of some general law—­when we show, in other words, that it does not stand alone.  But this does stand alone, and is admitted to stand alone.  We admit as much when we say that the mind is immaterial, and yet hold that it is related to the body.  We cannot, then, ask for an explanation of the relation.

But this does not mean that the reference of mental phenomena to the body is a meaningless expression.  We can point to those experiences of concomitance that we all have, distinguish them carefully from relations of another kind, and say:  This is what the word means, whether it be used by the plain man or by the man of science.

I have said above:  “If there is a brain change of a certain kind, there is the corresponding sensation.”  Perhaps the reader will feel inclined to say here:  If you can say as much as this, why can you not go a little farther and call the brain change the cause of the sensation?