t do not present themselves naturally to the healthy mind.
There is no thoughtful man who does not reflect sometimes and about
some things; but there are few who feel impelled to go over the whole
edifice of their knowledge and examine it with a critical eye from its
turrets to its foundations. In a sense, we may say that philosophical
thought is not natural, for he who is examining the assumptions upon
which all our ordinary thought about the world rests is no longer in
the world of the plain man. He is treating things as men do not
commonly treat them, and it is perhaps natural that it should appear to
some that, in the solvent which he uses, the real world in which we all
rejoice should seem to dissolve and disappear.

I have said that it is not the task of reflective thought, _in the
first instance_, to extend the limits of our knowledge of the world of
matter and of minds. This is true. But this does not mean that, as a
result of a careful reflective analysis, some errors which may creep
into the thought both of the plain man and of the scientist may not be
exploded; nor does it mean that some new extensions of our knowledge
may not be suggested.

In the chapters to follow I shall take up and examine some of the
problems of reflective thought. And I shall consider first those
problems that present themselves to those who try to subject to a
careful scrutiny our knowledge of the external world. It is well to
begin with this, for, even in our common experience, it seems to be
revealed that the knowledge of material things is a something less
vague and indefinite than the knowledge of minds.

II. PROBLEMS TOUCHING THE EXTERNAL WORLD

CHAPTER III

IS THERE AN EXTERNAL WORLD?

12. HOW THE PLAIN MAN THINKS HE KNOWS THE WORLD.--As schoolboys we
enjoyed Cicero's joke at the expense of the "minute philosophers."
They denied the immortality of the soul; he affirmed it; and he
congratulated himself upon the fact that, if they were right, they
would not survive to discover it and to triumph over him.

At the close of the seventeenth century the philosopher John Locke was
guilty of a joke of somewhat the same kind. "I think," said he,
"nobody can, in earnest, be so skeptical as to be uncertain of the
existence of those things which he sees and feels. At least, he that
can doubt so far (whatever he may have with his own thoughts) will
never have any controversy with me; since he can never be sure I say