moments,--what guarantee have we, then, that we are not always dreaming?
Therefore, our doubt must first of all be directed to the existence of
sense-objects. Nay, even mathematics must be suspected in spite of the
apparent certainty of its axioms and demonstrations, since controversy
and error are found in it also.

I doubt or deny, then, that the world is what it appears to be, that there
is a God, that external objects exist, that I have a body, that twice
two are four. One thing, however, it is impossible for me to bring into
question, namely, that I myself, who exercise this doubting function,
exist. There is one single point at which doubt is forced to halt--at the
doubter, at the self-existence of the thinker. I can doubt everything
except that I doubt, and that, in doubting, I am. Even if a superior being
sought to deceive me in all my thinking, he could not succeed unless I
existed, he could not cause me not to exist so long as I thought. To be
deceived means to think falsely; but that something is thought, no matter
what it be, is no deception. It might be true, indeed, that nothing at all
existed; but then there would be no one to conceive this non-existence.
Granted that everything may be a mistake; yet the being mistaken, the
thinking is not a mistake. Everything is denied, but the denier remains.
The whole content of consciousness is destroyed; consciousness itself, the
doubting activity, the being of the thinker, is indestructible. _Cogitatio
sola a me divelli nequit_. Thus the settled point of departure required for
knowledge is found in the _self-certitude of the thinking ego_. From the
fact that I doubt, _i.e._, think, it follows that I, the doubter, the
thinker, am. _Cogito, ergo sum_ is the first and most certain of all
truths.

The principle, "I think, therefore I am," is not to be considered a
deduction from the major premise, "Whatever thinks exists." It is rather