It appears somewhat easier for a man to have doubts upon this subject when he has fallen into the idealistic error of regarding the material world, which seems to be revealed to him, as nothing else than his “ideas” or “sensations” or “impressions.”  If we will draw the whole “telephone exchange” into the clerk, there seems little reason for not including all the subscribers as well.  If other men’s bodies are my sensations, may not other men’s minds be my imaginings?  But doubts may be felt also by those who are willing to admit a real external world.  How do we know that our inference to the existence of other minds is a justifiable inference?  Can there be such a thing as verification in this field?

For we must remember that no man is directly conscious of any mind except his own.  Men cannot exhibit their minds to their neighbors as they exhibit their wigs.  However close may seem to us to be our intercourse with those about us, do we ever attain to anything more than our ideas of the contents of their minds?  We do not experience these contents; we picture them, we represent them by certain proxies.  To be sure, we believe that the originals exist, but can we be quite sure of it?  Can there be a proof of this right to make the leap from one consciousness to another?  We seem to assume that we can make it, and then we make it again and again; but suppose, after all, that there were nothing there.  Could we ever find out our error?  And in a field where it is impossible to prove error, must it not be equally impossible to prove truth?

The doubt has seemed by no means a gratuitous one to certain very sensible practical men.  “It is wholly impossible,” writes Professor Huxley,[1] “absolutely to prove the presence or absence of consciousness in anything but one’s own brain, though by analogy, we are justified in assuming its existence in other men.”  “The existence of my conception of you in my consciousness,” says Clifford,[2] “carries with it a belief in the existence of you outside of my consciousness. . . .  How this inference is justified, how consciousness can testify to the existence of anything outside of itself, I do not pretend to say:  I need not untie a knot which the world has cut for me long ago.  It may very well be that I myself am the only existence, but it is simply ridiculous to suppose that anybody else is.  The position of absolute idealism may, therefore, be left out of count, although each individual may be unable to justify his dissent from it.”

These are writers belonging to our own modern age, and they are men of science.  Both of them deny that the existence of other minds is a thing that can be proved; but the one tells us that we are “justified in assuming” their existence, and the other informs us that, although “it may very well be” that no other mind exists, we may leave that possibility out of count.