premises of every conclusion, and to believe nothing but upon the
clearest evidence of reason; evidence so convincing that he could not by
any effort refuse to assent to it.

He has given us the detailed history of his doubts. He has told us how
he found that he could plausibly enough doubt of everything except of
his own existence. He pushed his scepticism to the verge of
self-annihilation. There he stopped; there in self, in his
consciousness, he found at last an irresistible fact, an irreversible
certainty.

Firm ground was discovered. He could doubt the existence of the external
world, and treat it as a phantasm; he could doubt the existence of a
God, and treat the belief as a superstition; but of the existing of his
thinking, doubting mind no sort of doubt was possible. He, the doubter,
existed if nothing else existed. The existence that was revealed in his
own consciousness was the primary fact, the first indubitable certainty.
Hence his famous "_Cogito, ergo sum_" ("I think, therefore I am").

It is somewhat curious, and, as an illustration of the frivolous verbal
disputes of philosophers, not a little instructive, that this celebrated
"_Cogito, ergo sum_," should have been frequently attacked for its
logical imperfection. It has been objected, from Gassendi downward, that
to say, "I think, therefore I am," is a begging of the question; since
existence has to be proved identical with thought. Certainly, if
Descartes had intended to prove his own existence by reasoning, he would
have been guilty of the _petitio principii_ Gassendi attributes to him,
viz., that the major premise, "that which thinks exists," is assumed,
not proved. But he did not intend this. What was his object? He has told
us that it was to find a starting-point from which to reason--to find an
irreversible certainty. And where did he find this? In his own
consciousness. Doubt as I may, I cannot doubt of my own existence,
because my very doubt reveals to me a something which doubts. You may
call this an assumption, if you will: I will point out the fact as one
above and beyond all logic; which logic can neither prove nor disprove;
but which must always remain an irreversible certainty, and as such a
fitting basis of philosophy.

I exist. No doubt can darken such a truth; no sophism can confute this
clear principle. This is a certainty, if there be none other. This is
the basis of all science. It is in vain to ask for a proof of that
which is