ful than powerful employing all his industry in misleading me. If there be such a demon, it may
be that all the things I see are only illusions of which he makes use as traps for my credulity.
There remains, however, something that I cannot doubt: no demon, however cunning, could
deceive me if I did not exist. I may have no body: this might be an illusion. But thought is
different. "While I wanted to think everything false, it must necessarily be that I who thought
was something; and remarking that this truth, I think, therefore I am, was so solid and so certain
that all the most extravagant suppositions of the sceptics were incapable of upsetting it, I judged
that I could receive it without scruple as the first principle of the philosophy that I sought." *
This passage is the kernel of Descartes's theory of knowledge, and contains what is most
important in his philosophy. Most philosophers since Descartes have attached importance to the
theory of knowledge, and their doing so is largely due to him. "I think, therefore I am" makes
mind more certain than matter, and my mind (for me) more certain than the minds of others.
There is thus, in all philosophy derived from Descartes, a tendency to subjectivism, and to
regarding matter as something only knowable, if at all, by inference from what is known of
mind. These two tendencies exist both in Continental idealism and in British empiricism--in the
former triumphantly, in the latter regretfully. There has been, in quite recent times, an attempt
to escape from this subjectivism by the philosophy known as instrumentalism, but of this I will
not speak at present. With this exception, modern philosophy has very largely accepted the
formulation of its problems from Descartes, while not accepting his solutions.
The reader will remember that Saint Augustine advanced an argument closely similar to the
cogito. He did not, however, give prominence to it, and the problem which it is intended to
solve occupied only a small part of his thoughts. Descartes's originality, therefore, should be
admitted, though it consists less in inventing the argument than in perceiving its importance.
Having now secured a firm foundation, Descartes sets to work to rebuild the edifice of
knowledge. The I that has been proved to exist
* The above argument, "I think, therefore I am" (cogito ergo sum), is known as Descartes's
cogito, and the process by which it is reached is cared "Cartesian doubt."