The difficulties presented by any philosophical doctrine are not always evident at once.  Descartes made no scruple of accepting the existence of an external world, and his example has been followed by a very large number of those who agree with his initial assumption that the mind knows immediately only its own ideas.

Preeminent among such we must regard John Locke, the English philosopher (1632-1704), whose classic work, “An Essay concerning Human Understanding,” should not be wholly unknown to any one who pretends to an interest in the English literature.

Admirably does Locke represent the position of what very many have regarded as the prudent and sensible man,—­the man who recognizes that ideas are not external things, and that things must be known through ideas, and yet holds on to the existence of a material world which we assuredly know.

He recognizes, it is true, that some one may find a possible opening for the expression of a doubt, but he regards the doubt as gratuitous; “I think nobody can, in earnest, be so skeptical as to be uncertain of the existence of those things which he sees and feels.”  As we have seen (section 12), he meets the doubt with a jest.

Nevertheless, those who read with attention Locke’s admirably clear pages must notice that he does not succeed in really setting to rest the doubt that has suggested itself.  It becomes clear that Locke felt so sure of the existence of the external world because he now and then slipped into the inconsistent doctrine that he perceived it immediately, and not merely through his ideas.  Are those things “which he sees and feels” external things?  Does he see and feel them directly, or must he infer from his ideas that he sees and feels them?  If the latter, why may one not still doubt?  Evidently the appeal is to a direct experience of material things, and Locke has forgotten that he must be a Lockian.

“I have often remarked, in many instances,” writes Descartes, “that there is a great difference between an object and its idea.”  How could the man possibly have remarked this, when he had never in his life perceived the object corresponding to any idea, but had been altogether shut up to ideas?  “Thus I see, whilst I write this,” says Locke,[1] “I can change the appearance of the paper, and by designing the letters tell beforehand what new idea it shall exhibit the very next moment, by barely drawing my pen over it, which will neither appear (let me fancy as much as I will), if my hand stands still, or though I move my pen, if my eyes be shut; nor, when those characters are once made on the paper, can I choose afterward but see them as they are; that is, have the ideas of such letters as I have made.  Whence it is manifest, that they are not barely the sport and play of my own imagination, when I find that the characters that were made at the pleasure of my own thought do not obey them; nor yet cease to be, whenever I shall fancy it; but continue to affect the senses constantly and regularly, according to the figures I made them.”