Now, in this chapter and in certain chapters to follow, I am going to take up and turn over, so that we may get a good look at them, some of the problems that have presented themselves to those who have reflected upon the world and the mind as they seem given in our experience.  I shall begin by asking whether it is not possible to doubt that there is an external world at all.

The question cannot best be answered by a jest.  It may, of course, be absurd to maintain that there is no external world; but surely he, too, is in an absurd position who maintains dogmatically that there is one, and is yet quite unable to find any flaw in the reasonings of the man who seems to be able to show that this belief has no solid foundation.  And we must not forget that the men who have thought it worth while to raise just such questions as this, during the last twenty centuries, have been among the most brilliant intellects of the race.  We must not assume too hastily that they have occupied themselves with mere trivialities.

Since, therefore, so many thoughtful men have found it worth while to ask themselves seriously whether there is an external world, or, at least, how we can know that there is an external world, it is not unreasonable to expect that, by looking for it, we may find in our common experience or in science some difficulty sufficient to suggest the doubt which at first strikes the average man as preposterous.  In what can such a doubt take its rise?  Let us see.

I think it is scarcely too much to say that the plain man believes that he does not directly perceive an external world, and that he, at the same time, believes that he does directly perceive one.  It is quite possible to believe contradictory things, when one’s thought of them is somewhat vague, and when one does not consciously bring them together.

As to the first-mentioned belief.  Does not the plain man distinguish between his ideas of things and the things themselves?  Does he not believe that his ideas come to him through the avenues of the senses?  Is he not aware of the fact that, when a sense is disordered, the thing as he perceives it is not like the thing “as it is”?  A blind man does not see things when they are there; a color-blind man sees them as others do not see them; a man suffering under certain abnormal conditions of the nervous system sees things when they are not there at all, i.e. he has hallucinations.  The thing itself, as it seems, is not in the man’s mind; it is the idea that is in the man’s mind, and that represents the thing.  Sometimes it appears to give a true account of it; sometimes it seems to give a garbled account; sometimes it is a false representative throughout—­there is no reality behind it.  It is, then, the idea that is immediately known, and not the thing; the thing is merely inferred to exist.

I do not mean to say that the plain man is conscious of drawing this conclusion.  I only maintain that it seems a natural conclusion to draw from the facts which he recognizes, and that sometimes he seems to draw the conclusion half-consciously.