Cogito ergo sum[a] is a Latin philosophical proposition by René Descartes usually translated into English as "I think, therefore I am". The phrase originally appeared in French as in his Discourse on the Method, so as to reach a wider audience than Latin would have allowed. It appeared in Latin in his later Principles of Philosophy. As Descartes explained, "we cannot doubt of our existence while we doubt...." A fuller form, penned by Antoine Léonard Thomas, aptly captures Descartes's intent: dubito, ergo cogito, ergo sum ("I doubt, therefore I think, therefore I am").[b] The concept is also sometimes known as the cogito.
This proposition became a fundamental element of Western philosophy, as it purported to form a secure foundation for knowledge in the face of radical doubt. While other knowledge could be a figment of imagination, deception, or mistake, Descartes asserted that the very act of doubting one's own existence served--at minimum--as proof of the reality of one's own mind; there must be a thinking entity--in this case the self--for there to be a thought.
Descartes first wrote the phrase in French in his 1637 Discourse on the Method. He referred to it in Latin without explicitly stating the familiar form of the phrase in his 1641 Meditations on First Philosophy. The earliest written record of the phrase in Latin is in his 1644 Principles of Philosophy, where, in a margin note (see below), he provides a clear explanation of his intent: "[W]e cannot doubt of our existence while we doubt". Fuller forms of the phrase are attributable to other authors.
The phrase first appeared (in French) in Descartes's 1637 Discourse on the Method in the first paragraph of its fourth part:
(French:) Ainsi, à cause que nos sens nous trompent quelquefois, je voulus supposer qu'il n'y avoit aucune chose qui fût telle qu'ils nous la font imaginer; Et parce qu'il y a des hommes qui se méprennent en raisonnant, même touchant les plus simples matières de Géométrie, et y font des Paralogismes, jugeant que j'étois sujet à faillir autant qu'aucun autre, je rejetai comme fausses toutes les raisons que j'avois prises auparavant pour Démonstrations; Et enfin, considérant que toutes les mêmes pensées que nous avons étant éveillés nous peuvent aussi venir quand nous dormons, sans qu'il y en ait aucune pour lors qui soit vraie, je me résolus de feindre que toutes les choses qui m'étoient jamais entrées en l'esprit n'étoient non plus vraies que les illusions de mes songes. Mais aussitôt après je pris garde que, pendant que je voulois ainsi penser que tout étoit faux, il falloit nécessairement que moi qui le pensois fusse quelque chose; Et remarquant que cette vérité, ,[c] étoit si ferme et si assurée, que toutes les plus extravagantes suppositions des Sceptiques n'étoient pas capables de l'ébranler, je jugeai que je pouvois la recevoir sans scrupule pour le premier principe de la Philosophie que je cherchais.[d]
(English:) Accordingly, seeing that our senses sometimes deceive us, I was willing to suppose that there existed nothing really such as they presented to us; And because some men err in reasoning, and fall into Paralogisms, even on the simplest matters of Geometry, I, convinced that I was as open to error as any other, rejected as false all the reasonings I had hitherto taken for Demonstrations; And finally, when I considered that the very same thoughts (presentations) which we experience when awake may also be experienced when we are asleep, while there is at that time not one of them true, I supposed that all the objects (presentations) that had ever entered into my mind when awake, had in them no more truth than the illusions of my dreams. But immediately upon this I observed that, whilst I thus wished to think that all was false, it was absolutely necessary that I, who thus thought, should be something; And as I observed that this truth, ,[c] was so certain and of such evidence that no ground of doubt, however extravagant, could be alleged by the Sceptics capable of shaking it, I concluded that I might, without scruple, accept it as the first principle of the philosophy of which I was in search.[e][f]
In 1641, Descartes published (in Latin) Meditations on first philosophy in which he referred to the proposition, though not explicitly as "cogito ergo sum" in Meditation II:
(Latin:) hoc pronuntiatum: ego sum, ego existo,[c] quoties a me profertur, vel mente concipitur, necessario esse verum.
In 1644, Descartes published (in Latin) his Principles of Philosophy where the phrase "ego cogito, ergo sum" appears in Part 1, article 7:
(Latin:) Sic autem rejicientes illa omnia, de quibus aliquo modo possumus dubitare, ac etiam, falsa esse fingentes, facilè quidem, supponimus nullum esse Deum, nullum coelum, nulla corpora; nosque etiam ipsos, non habere manus, nec pedes, nec denique ullum corpus, non autem ideò nos qui talia cogitamus nihil esse: repugnat enim ut putemus id quod cogitat eo ipso tempore quo cogitat non existere. Ac proinde haec cognitio, ego cogito, ergo sum,[c] est omnium prima & certissima, quae cuilibet ordine philosophanti occurrat.
(English:) While we thus reject all of which we can entertain the smallest doubt, and even imagine that it is false, we easily indeed suppose that there is neither God, nor sky, nor bodies, and that we ourselves even have neither hands nor feet, nor, finally, a body; but we cannot in the same way suppose that we are not while we doubt of the truth of these things; for there is a repugnance in conceiving that what thinks does not exist at the very time when it thinks. Accordingly, the knowledge, I think, therefore I am,[c] is the first and most certain that occurs to one who philosophizes orderly.[h]
Descartes's margin note for the above paragraph is:
(Latin:) Non posse à nobis dubitari, quin existamus dum dubitamus; atque hoc esse primum, quod ordine philosophando cognoscimus.
(English:) That we cannot doubt of our existence while we doubt, and that this is the first knowledge we acquire when we philosophize in order.[h]
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".[i]
A further expansion, dubito, ergo cogito, ergo sum--res cogitans ("...--a thinking thing") extends the cogito with Descartes's 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 ...".[j] This has been referred to as "the expanded cogito".
The phrase cogito ergo sum is not used in Descartes's 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 discover further truths 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 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's 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 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 ? 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.
The 8th century Hindu philosopher Adi Shankara wrote in a similar fashion, No one thinks, 'I am not', arguing that one's existence cannot be doubted, as there must be someone there to doubt. . The central idea of cogito ergo sum is also the topic of Mandukya Upanishad.
Spanish philosopher Gómez Pereira in his 1554 work De Inmortalitate Animae, published in 1749, wrote "nosco me aliquid noscere, et quidquid noscit, est, ergo ego sum" ("I know that I know something, anyone who knows exists, then I exist").
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" wherein the "it" could be an impersonal subject as in the sentence "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:
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 he entitles the Form of the Personal. "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, thus dissolving the integrity of our selves, and destroying any connection with reality. In order to formulate a more adequate cogito, Macmurray proposes the substitution of "I do" for "I think", ultimately leading to a belief in God as an agent to whom all persons stand in relation.
The father of Pragmatism, Charles Sanders Peirce, questioned whether a method of radical doubt provided an appropriate foundation for modern science and modern logic: "We cannot begin with complete doubt. We must begin with all the prejudices which we actually have when we enter upon the study of philosophy. ... A person may, it is true, in the course of his studies, find reason to doubt what he began by believing; but in that case he doubts because he has a positive reason for it, and not on account of the Cartesian maxim. Let us not pretend to doubt in philosophy what we do not doubt in our hearts."
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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