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