I think the answer to this question is rather clearly suggested in the sentence already quoted from Professor Huxley:  “It is wholly impossible 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.”

Here Professor Huxley admits that we have something like a proof, for he regards the inference as justified.  But he does not think that we have absolute proof—­the best that we can attain to appears to be a degree of probability falling short of the certainty which we should like to have.

Now, it should be remarked that the discredit cast upon the argument for other minds has its source in the fact that it does not satisfy a certain assumed standard.  What is that standard?  It is the standard of proof which we may look for and do look for where we are concerned to establish the existence of material things with the highest degree of certainty.

There are all sorts of indirect ways of proving the existence of material things.  We may read about them in a newspaper, and regard them as highly doubtful; we may have the word of a man whom, on the whole, we regard as veracious; we may infer their existence, because we perceive that certain other things exist, and are to be accounted for.  Under certain circumstances, however, we may have proof of a different kind:  we may see and touch the things themselves.  Material things are open to direct inspection.  Such a direct inspection constitutes absolute proof, so far as material things are concerned.

But we have no right to set this up as our standard of absolute proof, when we are talking about other minds.  In this field it is not proof at all.  Anything that can be directly inspected is not another mind.  We cannot cast a doubt upon the existence of colors by pointing to the fact that we cannot smell them.  If they could be smelt, they would not be colors.  We must in each case seek a proof of the appropriate kind.

What have we a right to regard as absolute proof of the existence of another mind?  Only this:  the analogy upon which we depend in making our inference must be a very close one.  As we shall see in the next section, the analogy is sometimes very remote, and we draw the inference with much hesitation, or, perhaps, refuse to draw it at all.  It is not, however, the kind of inference that makes the trouble; it is the lack of detailed information that may serve as a basis for inference.  Our inference to other minds is unsatisfactory only in so far as we are ignorant of our own minds and bodies and of other bodies.  Were our knowledge in these fields complete, we should know without fail the signs of mind, and should know whether an inference were or were not justified.

And justified here means proved—­proved in the only sense in which we have a right to ask for proof.  No single fact is known that can discredit such a proof.  Our doubt is, then, gratuitous and can be dismissed.  We may claim that we have verification of the existence of other minds.  Such verification, however, must consist in showing that, in any given instance, the signs of mind really are present.  It cannot consist in presenting minds for inspection as though they were material things.