But can all this be done in the absence of any first-hand knowledge of the things of which one is talking?
Remember that, if the psychologist is right, any external object, eye, ear, nerve, or brain, which we can perceive directly, is a mental complex, a something in the mind and not external at all.

How shall we prove that there are objects, ears, eyes, nerves, and brains,--in short, all the requisite mechanism for the calling into existence of sensations,--in an outer world which is not immediately perceived but is only inferred to exist?
I do not wish to be regarded as impugning the right of the psychologist to make the assumptions which he does, and to work as he does.

He has a right to assume, with the plain man, that there is an external world and that we know it.

But a very little reflection must make it manifest that he seems, at least, to be guilty of an inconsistency, and that he who wishes to think clearly should strive to see just where the trouble lies.
So much, at least, is evident: the man who is inclined to doubt whether there is, after all, any real external world, appears to find in the psychologist's distinction between ideas and things something like an excuse for his doubt.

To get to the bottom of the matter and to dissipate his doubt one has to go rather deeply into metaphysics.