common
experience. But even the plain man has heard of atoms and molecules;
and if he accepts the extension of knowledge offered him by the man of
science, he must admit that, whatever this apparently immediately
perceived external thing may be, it cannot be the external thing that
science assures him is out there in space beyond his body, and which
must be a very different sort of thing from the thing he seems to
perceive. The thing he perceives must, then, be _appearance_; and
where can that appearance be if not in his own mind?

The man who has made no study of philosophy at all does not usually
think these things out; but surely there are interrogation marks
written up all over his experience, and he misses them only because he
does not see clearly. By judiciously asking questions one may often
lead him either to affirm or to deny that he has an immediate knowledge
of the external world, pretty much as one pleases. If he affirms it,
his position does not seem to be a wholly satisfactory one, as we have
seen; and if he denies it, he makes the existence of the external world
wholly a matter of inference from the presence of ideas in the mind,
and he must stand ready to justify this inference.

To many men it has seemed that the inference is not an easy one to
justify. One may say: We could have no ideas of things, no sensations,
if real things did not exist and make an impression upon our senses.
But to this it may be answered: How is that statement to be proved? Is
it to be proved by observing that, when things are present and affect
the senses, there come into being ideas which represent the things?
Evidently such a proof as this is out of the question, for, if it is
true that we know external things only by inference and never
immediately, then we can never prove by observation that ideas and
things are thus connected. And if it is not to be proved by
observation, how shall it be proved? Shall we just assume it
dogmatically and pass on to something else? Surely there is enough in
the experience of the plain man to justify him in raising the question
whether he can certainly know that there is an external world.

13. THE PSYCHOLOGIST AND THE EXTERNAL WORLD.--We have seen just above
that the doubt regarding the existence of the world seems to have its
root in the familiar distinction between ideas and things, appearances
and the realities which they are supposed to represent. The
psychologist has much t