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 true that this general proposition is derived from the particular and earlier one.  I must first realize in my own experience that, as thinking, I exist, before I can reach the general conclusion that thought and existence are inseparable.  This fundamental truth is thus not a syllogism, but a not further deducible, self-evident, immediate cognition, a pure intuition—­sum cogitans.  Now, if my existence is revealed by my activity of thought, if my thought is my being, and the converse, if in me thought and existence are identical, then I am a being whose essence consists in thinking.  I am a spirit, an ego, a rational soul.  My existence follows only from my thinking, not from any chance action. Ambulo ergo sum would not be valid, but mihi videor or puto me ambulare, ergo sum.  If I believe I am walking, I may undoubtedly be deceived concerning the outward action (as, for instance, in dreams), but never concerning my inward belief. Cogitatio includes all the conscious activities of the mind, volition, emotion, and sensation, as well as representation and cognition; they are all modi cogitandi.  The existence of the mind is therefore the most certain of all things.  We know the soul better than the body.  It is for the present the only certainty, and every other is dependent on this, the highest of all.