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Doubt and certainty
Cogito ergo sum
Causal adequacy principle
Cartesian circle · Folium
Rule of signs · Cartesian diver
Res cogitans · Res extensa
Discourse on the Method
Meditations on First Philosophy
Principles of Philosophy
Passions of the Soul
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Gottfried Wilhelm Leibniz
Cogito ergo sum[a] (/ /, also //, //; Classical Latin: [ˈkoːɡitoː ˈɛrɡoː ˈsʊm], "I think, therefore I am", or better "I am thinking, therefore I exist") is a philosophical proposition by René Descartes. The simple meaning of the Latin phrase is that thinking about one’s existence proves—in and of itself—that an "I" exists to do the thinking; or, as Descartes explains, "[W]e cannot doubt of our existence while we doubt … ."
This proposition became a fundamental element of Western philosophy, as it was perceived to form a foundation for all knowledge. While other knowledge could be a figment of imagination, deception or mistake, the very act of doubting one's own existence arguably serves as proof of the reality of one's own existence, or at least of one's thought.
Descartes' original phrase, je pense, donc je suis (French pronunciation: [ʒə pɑ̃s dɔ̃k ʒə sɥi]), appeared in his Discourse on the Method (1637), which was written in French rather than Latin to reach a wider audience in his country than scholars. He used the Latin cogito ergo sum in the later Principles of Philosophy (1644).
The argument is popularly known in the English speaking world as "the cogito ergo sum argument" or, more briefly, as "the cogito".
Descartes first wrote the phrase in French in his 1637 Discours De la Méthode. He referred to it in Latin without explicitly stating the familiar form of the phrase in his 1641 Meditationes de Prima Philosophia. The earliest written record of the phrase in Latin is in his 1644 Principia Philosophiae, where he also provides a clear explanation of his intent in a margin note. Fuller forms of the phrase are due to other authors. [Formatting note: cogito variants in this section are highlighted in boldface to facilitate comparison; italics only as in originals.]
The phrase first appeared (in French) in Descartes' 1637 Discours de la Méthode (full title in English: Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences). From the first paragraph of Part IV:
In 1641, Descartes published (in Latin) Meditationes de Prima Philosophia (English: Meditations on first philosophy) in which he referred to the proposition, though not explicitly as "cogito ergo sum" in Meditation II:
In 1644, Descartes published (in Latin), Principia Philosophiae (English: Principles of Philosophy) where the phrase "ego cogito, ergo sum" appears in Part 1, article 7:
Descartes' margin note for the above paragraph is:
The proposition is sometimes given as "dubito, ergo cogito, ergo sum". This fuller form was penned by the eloquent French literary critic, Antoine Léonard Thomas, in an award-winning 1765 essay in praise of Descartes, where it appeared as "Puisque je doute, je pense; puisque je pense, j'existe." In English, this is "Since I doubt, I think; since I think I exist"; with rearrangement and compaction, "I doubt, therefore I think, therefore I am", or in Latin, "dubito, ergo cogito, ergo sum".[f]
A further expansion, "dubito, ergo cogito, ergo sum—res cogitans" ("…—a thinking thing") extends the cogito with Descartes' statement in the subsequent Meditation, "Ego sum res cogitans, id est dubitans, affirmans, negans, pauca intelligens, multa ignorans, volens, nolens, imaginans etiam et sentiens …", or, in English, "I am a thinking (conscious) thing, that is, a being who doubts, affirms, denies, knows a few objects, and is ignorant of many …".[g] This has been referred to as "the expanded cogito".
The phrase cogito ergo sum is not used in Descartes' Meditations on First Philosophy but the term "the cogito" is 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)
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 that it is impossible to doubt that he exists. Even if there were a deceiving god (or an evil demon), one's belief in their own existence would be secure, for there is no way one could be deceived unless one 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 deliberately and constantly deceives me. In that case, I, too, undoubtedly exist, if he deceives 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 claims only 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 does not say that his existence is necessary; he says that if he thinks, 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) or 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 similarly immediate clarity, distinctiveness, and self-evidence that presents 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 shall see — 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. Baruch Spinoza in "Principia philosophiae cartesianae" at its Prolegomenon identified "cogito ergo sum" the "ego sum cogitans" (I am a thinking being) as the thinking substance[disambiguation needed] with his ontological interpretation. It can also be considered that Cogito ergo sum is needed before any living being can go further in life".
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.)
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.
There have been a number of criticisms of the argument. One 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", a premise Descartes did not justify. In fact, he conceded that there would indeed be an extra premise needed, but denied that the cogito is a syllogism (see below).
To argue that the cogito is not a syllogism, one may call it self-evident that "Whatever has the property of thinking, exists". In plain English, it seems incoherent to actually doubt that one exists and is doubting. Strict skeptics maintain that only the property of 'thinking' is indubitably a property of the meditator (presumably, they imagine it possible that a thing thinks but does not exist). This countercriticism is similar to the ideas of Jaakko Hintikka, who offers a nonsyllogistic interpretation of cogito ergo sum. He claimed that one simply cannot doubt the proposition "I exist". To be mistaken about the proposition would mean something impossible: I do not exist, but I am still wrong.
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. Apparently, the first scholar who raised the problem was Pierre Gassendi. He points out that recognition that one has a set of thoughts does not imply that one is a particular thinker or another. Were we to move from the observation that there is thinking occurring to the attribution of this thinking to a particular agent, we would simply assume what we set out to prove, namely, that there exists a particular person endowed with the capacity for thought . In other words, the only claim that is indubitable here is the agent-independent claim that there is cognitive activity present The 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 criticized 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 suggested 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." David Hume claims that the philosophers who argue for a self that can be found using reason are confusing "similarity" with "identity". This means that the similarity of our thoughts and the continuity of them in this similarity do not mean that we can identify ourselves as a self but that our thoughts are similar.
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.
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 relativizing 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.
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.
The Danish philosopher Søren Kierkegaard provided a critical response to the cogito. Kierkegaard argues that the cogito already presupposes 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:
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.
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.
Kierkegaard argues that the value of the cogito is not its logical argument, but its psychological appeal: a thought must have something that exists to think the thought. 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 presupposed in order for thinking to occur, not that existence is concluded from that thinking.
The Scottish philosopher John Macmurray rejects the cogito outright in order to place action at the center of a philosophical system. "We must reject this, both as standpoint and as method. If this be philosophy, then philosophy is a bubble floating in an atmosphere of unreality."  The reliance on thought creates an irreconcilable dualism between thought and action in which the unity of experience is lost. In order to formulate a more adequate cogito, Macmurray proposes the substitution of "I do" for "I think".
Many philosophical skeptics and particularly radical skeptics would say that indubitable knowledge does not exist, is impossible, or has not been found yet, and would apply this criticism to the assertion that the "cogito" is beyond doubt.
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