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Cogito ergo sum (French: Je pense donc je suis; English: "I think, therefore I am"), sometimes misquoted as Dubito ergo cogito ergo sum (English: "I doubt, therefore I think, therefore I am"),[1] is a philosophical statement in Latin 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 (because, at the very least, there is an "I" who is doing the thinking).[2]

Descartes's original statement was "Je pense donc je suis," from his Discourse on Method (1637). He wrote it in French, not in Latin, thereby reaching 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.". 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. 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.


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 νόησις νοήσεως - nóesis noéseos) 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. Furthermore, in the Enchiridion Augustine attempts to refute skepticism by stating, "[B]y not positively affirming that they are alive, the skeptics ward off the appearance of error in themselves, yet they do make errors simply by showing themselves alive; one cannot err who is not alive. That we live is therefore not only true, but it is altogether certain as well" (Chapter 7 section 20). Another predecessor was Avicenna's "Floating Man" thought experiment on human self-awareness and self-consciousness.[3] Nevertheless, Descartes was original in expanding the idea as a defense against skepticism and using it as a foundation of his whole metaphysics.


There have been a number of criticisms of the argument. The first of the two under scrutiny here concerns the nature of the step from "I am thinking" to "I exist." The contention is that this is a syllogistic inference, for it appears to require the extra premise: "Whatever has the property of thinking, exists," and that extra premise must surely have been rejected at an earlier stage of the doubt.

It could be argued that "Whatever has the property of thinking, exists" is self-evident, and thus not subject to the method of doubt. This is because the instantiation principle states that: "Whatever has the property F, exists," but within the method of doubt, only the property of thinking is indubitably a property of the meditator. Descartes does not make use of this defense, however; as we have already seen, he responds to the criticism by conceding that there would indeed be an extra premise needed, but denying that the cogito is a syllogism. Jaakko Hintikka offered a non-syllogistic interpretation. "I exist" is immune to Descartes' method of doubt because it is impossible to be mistaken about one's own existence. If we do not exist then we cannot be mistaken, so we do.

Perhaps a more relevant contention is whether the "I" to which Descartes refers is justified. In Descartes, The Project of Pure Enquiry, Bernard Williams provides a history and full evaluation of this issue. The main objection, as presented by Georg Lichtenberg, is that rather than supposing an entity that is thinking, Descartes should have said: "thinking is occurring." That is, whatever the force of the cogito, Descartes draws too much from it; the existence of a thinking thing, the reference of the "I," is more than the cogito can justify. Friedrich Nietzsche criticised the phrase in that it presupposes that there is an "I", that there is such an activity as "thinking", and that "I" know what "thinking" is. He suggesting a more appropriate phrase would be "it thinks." In other words the "I" in "I think" could be similar to the "It" in "It is raining."

Williams provides a meticulous and exhaustive examination of this objection. He argues, first, that it is impossible to make sense of "there is thinking" without relativising it to something. However, this something cannot be Cartesian egos , because it is impossible to differentiate objectively between things just on the basis of the pure content of consciousness.

Williams' argument in detail

In addition to the preceding two arguments against the cogito, other arguments have been advanced by Bernard Williams. He claims, for example, that what we are dealing with when we talk of thought, or when we say "I am thinking," is something conceivable from a third-person perspective; namely objective "thought-events" in the former case, and an objective thinker in the latter.

The obvious problem is that, through introspection, or our experience of consciousness, we have no way of moving to conclude the existence of any third-personal fact, to conceive of which would require something above and beyond just the purely subjective contents of the mind.

Søren Kierkegaard's critique

The Danish philosopher Søren Kierkegaard provided a critical response to the cogito.[4] Kierkegaard argues that the cogito already pre-supposes the existence of "I", and therefore concluding with existence is logically trivial. Kierkegaard's argument can be made clearer if one extracts the premise "I think" into two further premises:

"x" thinks
I am that "x"
Therefore I think
Therefore I am

Where "x" is used as a placeholder in order to disambiguate the "I" from the thinking thing.[5]

Here, the cogito has already assumed the "I"'s existence as that which thinks. For Kierkegaard, Descartes is merely "developing the content of a concept", namely that the "I", which already exists, thinks.[6]

Kierkegaard argues that the value of the cogito is not its logical argument, but its psychological appeal: a thinking thing must have something that exists to think it. It is psychologically difficult to think "I do not exist". But as Kierkegaard argues, the proper logical flow of argument is that existence is already assumed or pre-supposed in order for thinking to occur, not that existence is concluded from that thinking.[7]


  1. ^ The extension "Dubito ergo... " was never used by Descartes himself. It appears in an anonymous introduction from 1765, signed by "Thomas", which was included in the Descartes edition by Victor Cousin and is accessible together with Descartes works at the Gutenberg Project.[1]. It is misleading as if Descartes argument would be based on doubt, whereas it is inherent in any mental activity whatsoever.
  2. ^ Baird, Forrest E.; Walter Kaufmann (2008). From Plato to Derrida. Upper Saddle River, New Jersey: Pearson Prentice Hall. ISBN 0-13-158591-6. 
  3. ^ Seyyed Hossein Nasr and Oliver Leaman (1996), History of Islamic Philosophy, p. 315, Routledge, ISBN 0415131596.
  4. ^ Kierkegaard, Søren. Philosophical Fragments. Trans. Hong, Princeton, 1985. p. 38-42.
  5. ^ Schönbaumsfeld, Genia. A Confusion of the Spheres. Oxford, 2007. p.168-170.
  6. ^ Kierkegaard, Søren. Philosophical Fragments. Trans. Hong, Princeton, 1985. p. 40.
  7. ^ Archie, Lee C, "Søren Kierkegaard, God's Existence Cannot Be Proved". Philosophy of Religion. Lander Philosophy, 2006.

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
  • S. Kierkegaard, Concluding Unscientific Postscript (Princeton, 1985) ISBN 978-0-691-02081-5
  • S. Kierkegaard, Philosophical Fragments (Princeton, 1985) ISBN 978-0-69-102036-5
  • B. Williams, Descartes, The Project of Pure Enquiry (Penguin, 1978) OCLC 4025089
  • Baird, Forrest E.; Walter Kaufmann (2008). From Plato to Derrida. Upper Saddle River, New Jersey: Pearson Prentice Hall. ISBN 0-13-158591-6. 

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