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 to say about ideas; and if sharpening and making
clear this distinction has anything to do with stirring up doubts, it
is natural to suppose that they should become more insistent when one
has exchanged the ignorance of everyday life for the knowledge of the
psychologist.

Now, when the psychologist asks how a given mind comes to have a
knowledge of any external thing, he finds his answer in the messages
which have been brought to the mind by means of the bodily senses. He
describes the sense-organs and the nervous connections between these