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.