nd 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 thin