(5) Such objections to parallelism as that cited above assume that the concomitance of which the parallelist speaks is analogous to physical concomitance. The chemist puts together a volume of hydrogen gas and a volume of chlorine gas, and the result is two volumes of hydrochloric acid gas. We regard it as essential to the result that there should be the two gases and that they should be brought together. But the fact that the chemist has red hair we rightly look upon as a concomitant phenomenon of no importance. The result would be the same if he had black hair or were bald. But this is not the concomitance that interests the parallelist. The two sorts of concomitance are alike only in the one point. Some phenomenon is regarded as excluded from the series of causes and effects under discussion. On the other hand, the difference between the two is all-important; in the one case, the concomitant phenomenon is an accidental circumstance that might just as well be absent; in the other, it is nothing of the sort; it _cannot_ be absent--the mental fact _must_ exist if the brain-change in question exists. It is quite possible that, on reading this list of points, one may be inclined to make two protests. First: Is a parallelism so carefully guarded as this properly called _parallelism_ at all? To this I answer: The name matters little. I have used it because I have no better term. Certainly, it is not the parallelism which is sometimes brought forward, and which peeps out from the citation from Clifford. It is nothing more than an insistence upon the truth that we should not treat the mind as though it were a material thing. If any one wishes to take the doctrine and discard the name, I have no objection. As so guarded, the doctrine is, I think, true. Second: If it is desirable to avoid the word "cause," in speaking of the relation of the mental and the physical, on the ground that otherwise we give the word a double sense, why is it not desirable to avoid the word "concomitance"? Have we not seen that the word is ambiguous? I admit the inconsistency and plead in excuse only that I have chosen the lesser of two evils. It is fatally easy to slip into the error of thinking of the mind as though it were material and had a place in the physical world. In using the word "concomitance" I enter a protest against this. But I have, of course, no right to use it without showing just what kind of concomitance I mean. [1] "First and Fundamental Truths," Book I, Part II, Chapter II. New York, 1889. [2] "Lectures and Essays," Vol. II, p. 57. London, 1879. CHAPTER X HOW WE KNOW THERE ARE OTHER MINDS