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.