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Cogito ergo sum

"Cogito, ergo sum" (I think, therefore I am), sometimes misquoted as Dubito, ergo cogito, ergo sum (Latin: "I doubt, therefore I think, therefore I am"), is a philosophical statement used by René Descartes, which became a foundational element of Western philosophy. The simple meaning of the phrase is that if someone is wondering whether or not he exists, that is in and of itself proof that he does exist.

Descartes's original statement was "Je pense donc je suis," from his Discourse on Method (1637). 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.", by which time it had become popularly known as 'the "Cogito Ergo Sum" argument'.


Although the idea expressed in Cogito ergo sum is widely attributed to Descartes, he was not the first to mention it. Plato spoke about the "knowledge of knowledge" (Greek noésis noéseós) and Aristotle explains the idea in full length:
But if life itself is good and pleasant (...) and if one who sees is conscious that he sees, one who hears that he hears, one who walks that he walks and similarly for all the other human activities there is a faculty that is conscious of their exercise, so that whenever we perceive, we are conscious that we perceive, and whenever we think, we are conscious that we think, and to be conscious that we are perceiving or thinking is to be conscious that we exist... (Nicomachean Ethics, 1170a25 ff.)

St. Augustine of Hippo in De Civitate Dei writes "Si […] fallor, sum" ("If I am mistaken, I am") (book XI, 26), and also anticipates modern refutations of the concept. Another predecessor was Avicenna's "Floating Man" thought experiment on human self-awareness and self-consciousness. Nevertheless, Descartes was original in expanding the idea as a defence against the growing scepticism of his time and using it as a foundation of his whole metaphysics.


The phrase Cogito ergo sum is not used in Descartes's most important work, the Meditations on First Philosophy, but the term "the cogito" is (often confusingly) used to refer to an argument from it. Descartes felt that this phrase, which he had used in his earlier Discourse, had been misleading in its implication that he was appealing to an inference, so he avoided the word ergo and wrote "that the proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind." (Meditation II.)

At the beginning of the second meditation, having reached what he considers to be the ultimate level of doubt — his argument from the existence of a deceiving god — Descartes examines his beliefs to see if any have survived the doubt. In his belief in his own existence he finds it is impossible to doubt that he exists. Even if there were a deceiving god (or an evil demon, the tool he uses to stop himself sliding back into ungrounded beliefs), his belief in his own existence would be secure, for how could he be deceived unless he existed in order to be deceived?

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 [or thought anything at all] then I certainly existed. But 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 that I am something. So, after considering everything very thoroughly, I must finally conclude that the proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind. (AT VII 25; CSM II 16–17)

There are three important notes to keep in mind here. First, he only claims the certainty of his own existence from the first-person point of view — he has not proved the existence of other minds at this point. This is something that has to be thought through by each of us for ourselves, as we follow the course of the meditations. Second, he is not saying that his existence is necessary; he is saying that if he's thinking, then necessarily he exists (see the instantiation principle). Third, this proposition "I am, I exist" is held true not based on a deduction (as mentioned above) nor on empirical induction, but on the clarity and self-evidence of the proposition.

Descartes does not use this first certainty, the cogito, as a foundation upon which to build further knowledge; rather, it is the firm ground upon which he can stand as he works to restore his beliefs. As he puts it:

Archimedes used to demand just one firm and immovable point in order to shift the entire earth; so I too can hope for great things if I manage to find just one thing, however slight, that is certain and unshakable. (AT VII 24; CSM II 16)

According to many of Descartes' specialists, including Étienne Gilson, the goal of Descartes in establishing this first truth is to demonstrate the capacity of his criterion — the immediate clarity and distinctiveness of self-evident propositions — to establish true and justified propositions despite having adopted a method of generalized doubt. As a consequence of this demonstration, Descartes considers science and mathematics to be justified to the extent that their proposals are established on a similar immediate clarity, distinctiveness, and self-evidence that present itself to the mind. The originality of Descartes' thinking, therefore, is not so much in expressing the cogito — a feat accomplished by other predecessors, as we have seen — but on using the cogito as demonstrating the most fundamental epistemological principle, that science and mathematics are justified by relying on clarity, distinctiveness, and self-evidence.


Further reading

  • W.E. Abraham, "Disentangling the Cogito", Mind 83:329 (1974)
  • Z. Boufoy-Bastick, Introducing 'Applicable Knowledge' as a Challenge to the Attainment of Absolute Knowledge , Sophia Journal of Philosophy, VIII (2005), pp 39–52.
  • R. Descartes (translated by John Cottingham), Meditations on First Philosophy, in The Philosophical Writings of Descartes vol. II (edited Cottingham, Stoothoff, and Murdoch; Cambridge University Press, 1984) ISBN 0-521-28808-8
  • G. Hatfield, Routledge Philosophy Guidebook to Descartes and the Meditations (Routledge, 2003) ISBN 0-415-11192-7
  • B. Williams, Descartes, The Project of Pure Enquiry (Penguin, 1978)
  • Baird, Forrest E.; Walter Kaufmann (2008). From Plato to Derrida. Upper Saddle River, New Jersey: Pearson Prentice Hall.

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