Cogito ergo sum (French: Je pense donc je suis; English: I think, therefore I am), often mistakenly stated as Dubito ergo cogito ergo sum (English: "I doubt, therefore I think, therefore I am"), is a philosophical Latin statement used by René Descartes, which became a fundamental element of Western philosophy. The simple meaning of the phrase is that if someone wonders whether or not he exists, that is, in and of itself, proof that he does exist (because, at the very least, there is an "I" who does the thinking). It forms the bedrock for all knowledge, because, while all things can be questioned as to whether they are from the realm of reality or from some figment of imagination (a dream, influence of a demon, etc.), the very act of doubting one's own existence serves as proof of the reality of one's own existence.
A common mistake is that people take the statement as proof that they, as a human person, exist. However, it is a severely limited conclusion that does nothing to prove that one's own body exists, let alone anything else that is perceived in the physical universe. It only proves that one's mind exists (that part of an individual that observes oneself doing the doubting). It does not rule out other possibilities, such as waking up to find oneself to be a butterfly who had dreamed of having lived a human life.
Descartes's original statement was "Je pense donc je suis," from his Discourse on Method (1637). He wrote it in French, not in Latin and thereby reached a wider audience in his country than that of scholars. He uses the Latin "Cogito ergo sum" in the later Principles of Philosophy (1644), Part 1, article 7: "Ac proinde hæc cognitio, ego cogito, ergo sum, est omnium prima & certissima, quæ cuilibet ordine philosophanti occurrat." (English: "This proposition, I think, therefore I am, is the first and the most certain which presents itself to whomever conducts his thoughts in order."). At that time, the argument had become popularly known in the English speaking world as 'the "Cogito Ergo Sum" argument', which is usually shortened to "Cogito" when referring to the principle virtually everywhere else.
The phrase Cogito ergo sum is not used in Descartes' Meditations on First Philosophy but the term "the cogito" is (often confusingly) used to refer to an argument from it. In the Meditations, Descartes phrases the conclusion of the argument as "that the proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind." (Meditation II.)
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