I have said (section 31) that Descartes (1596-1650) was fairly well acquainted with the functioning of the nervous system, and has much to say of the messages which pass along the nerves to the brain. The same sort of reasoning that leads the modern psychologist to maintain that we know only so much of the external world as is reflected in our sensations led him to maintain that the mind is directly aware of the ideas through which an external world is represented, but can know the world itself only indirectly and through these ideas. Descartes was put to sore straits to prove the existence of an external world, when he had once thus placed it at one remove from us. If we accept his doctrine, we seem to be shut up within the circle of our ideas, and can find no door that will lead us to a world outside. The question will keep coming back: How do we _know_ that, corresponding to our ideas, there are material things, if we have never perceived, in any single instance, a material thing? And the doubt here suggested may be reinforced by the reflection that the very expression "a material thing" ought to be meaningless to a man who, having never had experience of one, is compelled to represent it by the aid of something so different from it as ideas are supposed to be. Can material things really be to such a creature anything more than some complex of ideas? 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.