With this brief survey of some of the most interesting problems that confront the philosopher, I must content myself here.  Now let us turn and see how some of the fundamental problems treated in previous chapters have been approached by men belonging to certain well-recognized schools of thought.

And since it is peculiarly true in philosophy that, to understand the present, one must know something of the past, we shall begin by taking a look at the historical background of the types of philosophical doctrine to which reference is constantly made in the books and journals of the day.

[1] Ostwald, “Vorlesungen ueber Naturphilosophie,” s. 396.  Leipzig, 1902.

IV.  SOME TYPES OF PHILOSOPHICAL THEORY

CHAPTER XII

THEIR HISTORICAL BACKGROUND

48.  THE DOCTRINE OF REPRESENTATIVE PERCEPTION.—­We have seen in Chapter II that it seems to the plain man abundantly evident that he really is surrounded by material things and that he directly perceives such things.  This has always been the opinion of the plain man and it seems probable that it always will be.  It is only when he begins to reflect upon things and upon his knowledge of them that it occurs to him to call it in question.

Very early in the history of speculative thought it occurred to men, however, to ask how it is that we know things, and whether we are sure we do know them.  The problems of reflection started into life, and various solutions were suggested.  To tell over the whole list would take us far afield, and we need not, for the purpose we have in view, go back farther than Descartes, with whom philosophy took a relatively new start, and may be said to have become, in spirit and method, at least, modern.

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?