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"I think, therefore I am" (the cogito)
1637· Idea or argument· Knowledge, logic & science
Well documentedSolid records, texts or physical evidence.
Descartes' claim that even if he doubts everything, the very act of doubting proves that he exists.
See it on the Philosophy map →The idea
Idea: I can doubt everything except that I am thinking, so I must exist. Reasoning: imagine an evil demon deceiving you about the world, your body, even that 2 + 3 = 5. To be deceived, you must exist. The French 'je pense, donc je suis' appears in 1637; the Latin 'cogito, ergo sum' in 1644. Main objection: Lichtenberg and others said it proves only that thinking is happening, not that a lasting 'I' does it.
Why it matters
It made the individual mind the starting point of knowledge, and with it the problem of how we can know anything outside our own heads.
Think of it like
Like a title search that ends at the one deed no one can contest; every other claim is built from there.
Where it comes from
In Descartes' Discourse on the Method (1637) and Principles of Philosophy (1644); the Meditations (1641) gives the fuller argument with printed objections and replies.
Still debated
Whether it is an inference ('therefore') or a direct intuition is debated, as is whether it proves a self or only thoughts.
Connections (4)
▸Augustine of Hippo anticipated "I think, therefore I am" (the cogito)Well documented
The connection
About 1,200 years before Descartes, Augustine answered skeptics with 'If I am mistaken, I exist' (si fallor, sum): even being deceived proves the one deceived is real. Descartes' cogito makes the same move, but uses it as the foundation of all knowledge rather than one argument among several.
When and how
Augustine's City of God, Book 11 (written c. 413–426) and On the Trinity; Descartes' Discourse (1637) and Meditations (1641).
How we know
Arnauld and Mersenne pointed out the parallel to Descartes, and his 1640 letter replying to the comparison survives.
Debates and open questions
Descartes said he had not read the passage and that Augustine used it differently; whether he absorbed it indirectly through his schooling is unknown.
▸René Descartes proposed "I think, therefore I am" (the cogito)Well documented
The connection
Searching for one truth beyond doubt, Descartes doubted his senses, his body and even mathematics, imagining an all-powerful deceiver. He found that the act of doubting proved his own existence as a thinking thing, and made this cogito the first stone of his whole system.
When and how
First in French in the Discourse on the Method (Leiden, 1637), then argued fully in the Meditations (Paris, 1641) and put in Latin in the Principles (1644).
How we know
Descartes' texts state it repeatedly, and Hobbes, Gassendi and Arnauld debated it in the printed Objections and Replies.
Debates and open questions
Whether he truly built on it alone, or smuggled in other assumptions such as what 'thinking' and 'existing' mean, is debated.
▸"I think, therefore I am" (the cogito) offered a doubt-proof foundation for What can we know?Mostly agreed
The connection
Plato asked what knowledge is and how it differs from belief. Descartes answered the skeptics with one certainty, the cogito, then tried to build all knowledge on it, like a building on bedrock. This 'foundationalism' set the agenda of epistemology, the theory of knowledge, for centuries.
When and how
Descartes' Discourse (1637) and Meditations (1641); Plato's Theaetetus (c. 369 BC) had framed the older question.
How sure are we?
Epistemology textbooks treat Cartesian foundationalism as a central answer, and the empiricists, Kant and 20th-century philosophers all responded to it explicitly.
Debates and open questions
Many now doubt that knowledge needs certain foundations at all; coherence and reliability theories offer rival pictures.
▸"I think, therefore I am" (the cogito) was reworked by Husserl and phenomenologyWell documented
The connection
Husserl saw Descartes as phenomenology's ancestor: both start from the certainty of one's own consciousness. But Husserl reworked the cogito. Instead of using it to prove God and the outside world, he 'bracketed' the world and studied the structure of experience itself, stressing that every thought is about something.
When and how
In lectures at the Sorbonne in Paris in 1929, published in French in 1931 as Cartesian Meditations.
How we know
The title and text build explicitly on Descartes' Meditations; Husserl says phenomenology might almost be called 'a neo-Cartesianism'.
Debates and open questions
Whether Husserl stayed trapped in Descartes' isolated mind, as Heidegger and others charged, or escaped it through his later idea of the shared 'life-world' is debated.