Cogito ergo sum

"Cogito ergo sum," Latin for "I think, therefore I am," is Rene Descartes' most famous catchphrase and one of the most famous statements in all of philosophy. This statement is emblematic of a turning point in the history of philosophy, making the certainty of individual consciousness the new foundation of knowledge.

The cogito, as the statement is often called, occurs most famously in the Discourse on Method, Part 4, but not, as is sometimes thought, in the main text of the Meditations on First Philosophy. Still, the more complete explanation of the meaning of the statement can be found in Meditation Two.

The general context of the statement is this: Descartes takes certain knowledge as his aim, and holds that knowledge is not certain unless it is "indubitable," or not capable of being doubted. In his quest for knowledge, then, he undertakes to doubt what he believes, in a radical way--what has been called "Cartesian doubt" or "hyperbolic doubt." He finds reasons to think it is possible (even if very improbable) that his sense-perception is misleading (he could be dreaming) and that his beliefs in the most basic truths of mathematics are wrong (he could be deceived by an evil genius or demon). Still, even if he doubts such obvious things, he cannot doubt that he exists:

...am not I, at least, something? But I have just said that I have no senses and no body [i.e., I doubt that I have any]. This is the sticking point: what follows from this? Am I not so bound up with a body and with senses that I cannot exist without them? But I have convinced myself that there is absolutely nothing in the world, no sky, no earth, no minds, no bodies. Does it now follow that I too do not exist? No: if I convinced myself of something then I certainly existed. But [I posit, as part of my doubts] there is a deceiver of supreme power and cunning who is deliberately and constantly deceiving me. In that case I too undoubtedly exist, if he is deceiving me; and let him deceive me as much as he can, he will never bring it about that I am nothing so long as I think I am something. So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.

Philosophers often debate whether the cogito is an argument, that is, whether "I think" is a premise, from which "I exist" is inferred as a conclusion. A careful reading of the above passage should make it clear why philosophers often say that the statement does not express an argument, even if (in the Discourse, published four years before the Meditations) Descartes' sentiment is expressed with the word "therefore" (ergo). He says that whenever he conceives of the proposition that he exists, it is "necessarily true"; merely conceiving of the proposition that he exists is enough to make him think that it must be true. This does not mean that he infers it from a prior proposition, that he thinks, and that he believes he knows that he thinks more certainly than that he exists.

There is a long history of criticism and interpretation of this single statement. Among the earliest is by Thomas Hobbes, Descartes' contemporary, who in the Third Set of Objections with Replies,