What did Descartes mean by 'I think, therefore I am'?

Descartes used the phrase to name a first certainty that survives even the most extreme doubt. Even if a powerful deceiver is tricking him about everything else, he must exist in order to be deceived. The attempt to doubt his own thinking undermines itself, so the existence of the thinker is secure whenever he thinks it. The Latin is cogito, ergo sum, and one commentary suggests a clearer rendering is 'I am thinking, therefore I exist'. [1] [2] [3]

Descartes first published the idea in French, as je pense, donc je suis, in his 1637 Discourse on the Method, aiming at a wider audience than Latin allowed. The Latin form appeared later in his Principles of Philosophy. In the Meditations, the key passage is the Second Meditation's 'I am, I exist', and the exact formula does not expressly appear there. Descartes called the cogito the first and most certain item to occur to anyone who philosophizes in an orderly way. [2] [1]

The idea was also anticipated by Augustine, according to the Stanford Encyclopedia's entry on self-consciousness. [4]

Descartes' method was to doubt whatever could be doubted. The existence of his body can be doubted, since he might be dreaming, but the existence of his thinking cannot. Several features matter. The claim must be in the first person, because 'Descartes thinks' is not unshakably certain for me. It must be in the present tense, because remembering that I thought last Tuesday does not survive doubt, and nor does the claim that I will keep existing. It must also concern thinking in a broad sense, including doubting, willing or imagining, rather than bodily activity such as walking, though 'it seems I am walking' would work. [1]

The claim is also modest about what the 'I' is. Right after stating it, the meditator says he does not yet understand what this 'I' is. The cogito establishes that I exist as a thinking thing, whatever that turns out to be, and does not by itself settle the nature of the mind or its relation to the body. [1]

The main ongoing debate is whether the cogito is an inference or an intuition. Jaakko Hintikka argued in 1962 that it is non-inferential and performative. On his view, 'I exist' does not follow logically from 'I think'; rather, the act of thinking 'I do not exist' is existentially incoherent, so the thought verifies itself. A passage in Descartes' Second Replies seems to support this, saying that one does not deduce existence from thought by a syllogism but sees it by a simple intuition of the mind. [1]

Others reply that the passage rejects only a syllogism, not inference in general, as Margaret Wilson observes. Inferential readings can also allow that the cogito is self-evident, just as modus ponens is widely held to be self-evident yet contains an inference. Edwin Curley notes that Descartes calls the whole formula a truth, a proposition and a conclusion alike, and Anthony Kenny says that what is intuited from one point of view is deduced from another. The source describes the question as ongoing and does not report a consensus. [1]

The cogito became a fundamental element of Western philosophy because it offered a certain foundation for knowledge in the face of radical doubt. For Descartes it played a foundational role in his epistemological project, and later accounts treated his emphasis on consciousness of one's own thinking as marking mind off from body. [2] [4] [5]