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. I merely wish to show just here that the doubt is not a gratuitous one, but is really suggested to the thoughtful mind by a reflection upon our experience of things. And, as we are all apt to think that the man of science is less given to busying himself with useless subtleties than is the philosopher, I shall, before closing this chapter, present some paragraphs upon the subject from the pen of a professor of mathematics and mechanics. 14. THE "TELEPHONE EXCHANGE."--"We are accustomed to talk," writes Professor Karl Pearson,[1] "of the 'external world,' of the 'reality' outside us. We speak of individual objects having an existence independent of our own. The store of past sense-impressions, our thoughts and memories, although most probably they have beside their psychical element a close correspondence with some physical change or impress in the brain, are yet spoken of as _inside_ ourselves. On the other hand, although if a sensory nerve be divided anywhere short of the brain, we lose the corresponding class of sense impression, we yet speak of many sense-impressions, such as form and texture, as existing outside ourselves. How close then can we actually get to this supposed world outside ourselves? Just as near but no nearer than the brain terminals of the sensory nerves. We are like the clerk in the central telephone exchange who cannot get nearer to his customers than his end of the telephone wires. We are indeed worse off than the clerk, for to carry out the analogy properly we must suppose him _never to have been outside the telephone exchange, never to have seen a customer or any one like a customer--in short, never, except through the telephone wire, to have come in contact with the outside universe_. Of that 'real' universe outside himself he would be able to form no direct impression; the real universe for him would be the aggregate of his constructs from the messages which were caused by the telephone wires in his office. About those messages and the ideas raised in his mind by them he might reason and draw his inferences; and his conclusions would be correct--for what? For the world of telephonic messages, for