Cogito, ergo sum
Cogito, ergo sum[a] is a philosophical statement that was made in Latin by René Descartes, usually translated into English as "I think, therefore I am".[b] The phrase originally appeared in French as je pense, donc je suis 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 it, "we cannot doubt of our existence while we doubt." A fuller version, articulated by Antoine Léonard Thomas, aptly captures Descartes's intent: dubito, ergo cogito, ergo sum ("I doubt, therefore I think, therefore I am").[c][d] The dictum is also sometimes referred to as the cogito.
Descartes's statement became a fundamental element of Western philosophy, as it purported to provide a certain 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.
One common critique of the dictum is that it presupposes that there is an "I" which must be doing the thinking. According to this line of criticism, the most that Descartes was entitled to say was that "thinking is occurring", not that "I am thinking".
In Descartes's writingsEdit
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.
Discourse on the MethodEdit
The phrase first appeared (in French) in Descartes' 1637 Discourse on the Method in the first paragraph of its fourth part:
Ainsi, à cause que nos sens nous trompent quelquefois, je voulus supposer qu'il n'y avait 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'étais sujet à faillir autant qu'aucun autre, je rejetai comme fausses toutes les raisons que j'avais 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 raison pour lors qui soit vraie, je me résolus de feindre que toutes les choses qui m'étaient jamais entrées en l'esprit n'étaient non plus vraies que les illusions de mes songes. Mais aussitôt après je pris garde que, pendant que je voulais ainsi penser que tout était faux, il fallait nécessairement que moi qui le pensais fusse quelque chose; Et remarquant que cette vérité, je pense, donc je suis,[e] était si ferme et si assurée, que toutes les plus extravagantes suppositions des Sceptiques n'étaient pas capables de l'ébranler, je jugeai que je pouvais la recevoir sans scrupule pour le premier principe de la Philosophie que je cherchais.[f][g]
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, I think, therefore I am,[e] 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.[h][i]
Meditations on First PhilosophyEdit
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:
Principles of PhilosophyEdit
In 1644, Descartes published (in Latin) his Principles of Philosophy where the phrase "ego cogito, ergo sum" appears in Part 1, article 7:
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,[e] est omnium prima & certissima, quae cuilibet ordine philosophanti occurrat.[l]
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,[m] I think, therefore I am,[e] is the first and most certain that occurs to one who philosophizes orderly.[n]
Descartes's margin note for the above paragraph is:
Non posse à nobis dubitari, quin existamus dum dubitamus; atque hoc esse primum, quod ordine philosophando cognoscimus.
That we cannot doubt of our existence while we doubt, and that this is the first knowledge we acquire when we philosophize in order.[n]
The Search for TruthEdit
Descartes, in a lesser-known posthumously published work dated as written ca. 1647 and titled La Recherche de la Vérité par La Lumiere Naturale (The Search for Truth by Natural Light),[o] wrote:
… [S]entio, oportere, ut quid dubitatio, quid cogitatio, quid exsistentia sit antè sciamus, quàm de veritate hujus ratiocinii : dubito, ergo sum, vel, quod idem est, cogito, ergo sum[e] : plane simus persuasi.
… [I feel that] it is necessary to know what doubt is, and what thought is, [what existence is], before we can be fully persuaded of this reasoning — I doubt, therefore I am — or what is the same — I think, therefore I am.[p]
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" ('Since I doubt, I think; since I think, I exist'). With rearrangement and compaction, the passage translates to "I doubt, therefore I think, therefore I am," or in Latin, "dubito, ergo cogito, ergo sum."[q]
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…" ("I am a thinking [conscious] thing, that is, a being who doubts, affirms, denies, knows a few objects, and is ignorant of many…").[r] This has been referred to as "the expanded cogito."[s]
Following John Lyons (1982), Vladimir Žegarac notes, "The temptation to use the simple present is said to arise from the lack of progressive forms in Latin and French, and from a misinterpretation of the meaning of cogito as habitual or generic" (cf. gnomic aspect). Also following Lyons, Ann Banfield writes, "In order for the statement on which Descartes's argument depends to represent certain knowledge,… its tense must be a true present—in English, a progressive,… not as 'I think' but as 'I am thinking, in conformity with the general translation of the Latin or French present tense in such nongeneric, nonstative contexts." Or in the words of Simon Blackburn, "Descartes’s premise is not ‘I think’ in the sense of ‘I ski’, which can be true even if you are not at the moment skiing. It is supposed to be parallel to ‘I am skiing’."
Fumitaka Suzuki (2012) writes "Taking consideration of Cartesian theory of continuous creation, which theory was developed especially in the Meditations and in the Principles, we would assure that 'I am thinking, therefore I am/exist' is the most appropriate English translation of 'ego cogito, ergo sum'."
The similar translation “I am thinking, therefore I exist” of Descartes's correspondence in French (“je pense, donc je suis”) appears in The Philosophical Writings of Descartes by Cottingham et al. (1988).:247
As put succinctly by Krauth (1872), "That cannot doubt which does not think, and that cannot think which does not exist. I doubt, I think, I exist."
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)[v]
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)[v]
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.
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.)
In the late sixth or early fifth century BC, Parmenides is quoted as saying "For to be aware and to be are the same" (B3). Augustine of Hippo in De Civitate Dei (book XI, 26) writes "If I am mistaken, I am" ("Si…fallor, sum"), and also anticipates modern refutations of the concept. Furthermore, in the Enchiridion (ch. 7, sec. 20), 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." In 1640 correspondence, Descartes thanked two colleagues for drawing his attention to Augustine and notes similarity and difference. :159, 161
The 8th century Hindu philosopher Adi Shankara wrote, in a similar fashion, that 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, & quidquid noscit, est, ergo ego sum" ('I know that I know something, anyone who knows exists, then I exist').
Use of "I"Edit
In Descartes, The Project of Pure Enquiry, Bernard Williams provides a history and full evaluation of this issue. The first to raise the "I" 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."
The Danish philosopher Søren Kierkegaard calls the phrase a tautology in his Concluding Unscientific Postscript.:38–42 He 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 the premises "'x' thinks" and "I am that 'x'", 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.:40 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.
Bernard Williams claims 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. 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.
This certainty, that "I myself am in that I will die," is the basic certainty of Dasein itself. It is a genuine statement of Dasein, while cogito sum is only the semblance of such a statement. If such pointed formulations mean anything at all, then the appropriate statement pertaining to Dasein in its being would have to be sum moribundus [I am in dying], moribundus not as someone gravely ill or wounded, but insofar as I am, I am moribundus. The MORIBUNDUS first gives the SUM its sense.
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.
- Descartes wrote this phrase only once, in a posthumously published lesser-known work. It appeared there mid-sentence, uncapitalized, and with a comma. (Commas were not used in classical Latin but were a regular feature of scholastic Latin. Most modern reference works show it with a comma, but it is often presented without a comma in academic work and in popular usage.) In the primary source, Descartes's Principia Philosophiae, the proposition appears as ego cogito, ergo sum.
- Some sources offer "I am thinking, therefore I am" as a 'better' translation. (See § Translation.)
- The dubito is often mistakenly attributed to Descartes. (See Other forms.)
- In the posthumously published work cited in the first footnote above, Descartes wrote “dubito, ergo sum, vel, quod idem est, cogito, ergo sum" ("I doubt, therefore I am — or what is the same — I think, therefore I am"). (See The Search for Truth.)
- Formatting note: cogito variants in this section are highlighted in boldface to facilitate comparison; italics are used only as in originals.
- Formatting note: Capitalization as in original; spelling updated from Middle French to Modern French.
- See original Discours manuscript here.
- This translation, by Veitch in 1850, is modified here as follows: Veitch's "I think, hence I am” is changed to the form by which it is currently best known in English, "I think, therefore I am", which appeared in the Haldane and Ross 1911 translation, and as an isolated attributed phrase previously, e.g., in Sullivan (1794); in the preceding line, Veitch's "I, who thus thought, should be somewhat” is given here as "… should be something" for clarity (in accord with other translations, e.g., that of Cress); and capitalization was reverted to conform to Descartes's original in French.
- The 1637 Discours was translated to Latin in the 1644 Specimina Philosophiae but this is not referenced here because of issues raised regarding translation quality.
- See original Meditiations manuscript here.
- This combines, for clarity and to retain phrase ordering, the Cress and Haldane translations of the Meditations with the translation of the “ego cogito, ergo sum” phrase by Suzuki (see § Translation).
- See original Principia manuscript here.
- A 1647 French translation, published with Descartes’s enthusiastic approval, substituted 'conclusion' for 'knowledge'.
- Translation from The Principles of Philosophy at Project Gutenberg.
- Titled Inquisitio Veritatis per Lumen Naturale in a 1683 compendium of posthumously published works.
- Translation by Hallam, with additions for completeness.
- The 1765 work, Éloge de René Descartes, by Antoine Léonard Thomas, was awarded the 1765 Le Prix De L'académie Française and republished in the 1826 compilation of Descartes's work, Oeuvres de Descartes by Victor Cousin. The French text is available in more accessible format at Project Gutenberg. The compilation by Cousin is credited with a revival of interest in Descartes.
- This translation by Veitch is the first English translation from Descartes as "I am a thinking thing".
- Martin Schoock, who in the 1642–43 controversy between Descartes and Gisbertus Voetius, fiercely attacked Descartes and his philosophy in an essay, wrote cogito, ergo sum, res cogitans and cogito, inquiro, dubito ergo sum as well as cogito, ergo sum (multiple times) in his 1652 De Scepticismo.
- The tense of je pense is marked indicatif présent by e.g., conjugation.com; cōgitō is indicative active present per e.g., Wiktionary.
- Krauth is not explicitly acknowledged as author of this article, but is so identified the following year by Garretson.
- AT refers to Adams and Tannery; CSM II to Cottingham, Stoothoff, and Murdoch; CSMK III to Cottingham, Stoothoff, Murdoch, and Kenny
- Burns, William E. (2001). The scientific revolution: an encyclopedia. Santa Barbara, California: ABC-CLIO. p. 84. ISBN 978-0-87436-875-8.
- " Cogito". Oxford Dictionaries. Oxford University Press.
- Monte, Jonas (2015). "Sum, Ergo Cogito: Nietzsche Re-orders Decartes" (PDF). Aporia.byu.edu. BYU. Archived from the original (PDF) on 4 August 2016. Retrieved 17 September 2016.
- Veitch, John (1850). Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences, by Descartes. Edinburgh: Sutherland and Knox. pp. 74–5.
- Descartes, René (1911). The Philosophical Works of Descartes, rendered into English. Translated by Elizabeth S. Haldane and G.R.T. Ross. Cambridge University Press.
- Richard Joseph Sullivan (1794). A View of Nature, in Letters to a Traveller among the Alps, with Reflections on Atheistical Philosophy now exemplified in France. London: printed for T. Becket. p. 129.
- Descartes, René (1986). Discourse on Method and Meditations on First Philosophy. Translated by Donald A. Cress. p. 65. ISBN 978-1-60384-551-9.
- Descartes, René (1644). Specimina philosophiae. Ludovicus Elzevirius. p. 30.
- Vermeulen, Corinna Lucia (2006). René Descartes, Specimina philosophiae. Introduction and Critical Edition (Dissertation, Utrecht University). hdl:1874/23451.
- Descartes, René (1960). Meditations on first philosophy. Translated by Elizabeth S. Haldane. p. 29. ISBN 978-1-61536-207-3.
- Descartes (1647). Principes de la philosophie. Translated by Picot, Abbé Claude. Paris. ISBN 9782711622313.
- Miller, Valentine Roger; Miller, Reese P. (1983). Descartes, René. Principles of Philosophy. Translated, with explanatory notes. pp. xi, 5. ISBN 978-90-277-1754-2.
- Gouhier, Henri (1924), La pensée religieuse de Descartes, p. 319
- Adam, Charles; Tannery, Paul, eds. (1901), "La Recherche de la Vérité par La Lumiere Naturale", Oeuvres de Descartes, X, p. 535
- Descartes, René (1683), Regulae ad Directionem Ingenii ut et Inquisitio Veritatis per Lumen Naturale, p. 86
- Hallam, Henry (1843), Introduction to the Literature of Europe in the 15th, 16th, and 17th Centuries, II (2nd ed.), p. 451
- Thomas, Antoine Léonard (1765). Éloge de René Descartes. E. van Harrevelt. p. 32.
- Cousin, Victor (1824). Oeuvres de Descartes.
- The Edinburgh Review for July, 1890 … October, 1890. Leonard Scott Publication Co. 1890. p. 469.
- Bohemia, Princess Elisabeth of; Descartes, René (2007-11-01). The Correspondence between Princess Elisabeth of Bohemia and René Descartes. University of Chicago Press. ISBN 978-0-226-20444-4.
- Veitch, John (1880). The Method, Meditations and Selections from the Principles of René Descartes (7th ed.). Edinburgh: William Blackwood and Sons. p. 115.
- Kline, George L. (1967). "Randall's Interpretation of the Philosophies of Descartes, Spinoza, and Leibniz". In John Peter Anton (ed.). Naturalism and Historical Understanding. SUNY Press. p. 85.
- Schoockius, Martinus (1643), Admiranda Methodus Novae Philosophiae Renati Des Cartes
- Schoockius, Martinus (1652), De Scepticismo, p. 87
- Pope, Rob (2013). Textual Intervention: Critical and Creative Strategies for Literary Studies. Routledge. p. 35. ISBN 9781135083281.
- Dunlap, J. R. (2006). An Answer Key to A Primer of Ecclesiastical Latin: A Supplement to the Text by John F. Collins. CUA Press. p. 18. ISBN 9780813214696.
- Lyons, J. (1982). "Deixis and subjectivity: Loquor, ergo sum?". In Jarvella, Rovert J.; Klein, Wolfgang (eds.). Speech, place, and action: Studies in deixis and related topics. pp. 101–224.
- Žegarac, Vladimir (1991). Tense, aspect and relevance (PDF) (PhD thesis). University of London. pp. 84, 85.
- Banfield, A. (1998). "The Name of the Subject: The "il"?". Yale French Studies (93): 133–174. doi:10.2307/3040735. JSTOR 3040735.
- Simon Blackburn (1999). Think: A compelling introduction to philosophy. Oxford University Press. ISBN 9780199769841.
“am thinking, therefore”.
- Suzuki, Fumitaka (2012). "The Cogito Proposition of Descartes and Characteristics of His Ego Theory". Aporia.byu.edu. Bulletin of Aichi Univ. of Education. Archived from the original on 10 May 2018. Retrieved 6 May 2018.
- The Philosophical Writings of Descartes. III. Translated by Cottingham, J.; Stoothoff, R.; Kenny, A.; Murdoch, D. Cambridge University Press. 1991. ISBN 9780521423502.
- Krauth, Charles Porterfield (1872). "Notes in Class — Descartes". The Penn Monthly. University Press Company. 3: 11.
- James Edmund Garretson (1873). Thinkers and Thinking. J.B. Lippincott & Company. p. 182.
descartes he affirmed thinking.
- The Philosophical Writings of Descartes. II. Translated by Cottingham, J.; Stoothoff, R.; Kenny, A.; Murdoch, D. Cambridge University Press. 1984. ISBN 978-0-521-28808-8.
- Self, Reason, and Freedom: A New Light on Descartes' Metaphysics Andrea Christofidou; chapter 2
- Nasr, Seyyed Hossein, and Oliver Leaman. 1996. History of Islamic Philosophy. Routledge. p. 315. ISBN 0-415-13159-6.
- Radhakrishnan, Sarvepalli. 1948. Indian Philosophy II. George Allen & Unwin Ltd. p. 476.
- Pereira, Gómez.  1749. Antoniana Margarita: "De Immortalitate Animae". p. 277.
- López, Modesto Santos. 1986. "Gómez Pereira, médico y filósofo medinense." In Historia de Medina del Campo y su Tierra, volumen I: Nacimiento y expansión, edited by E. L. Sanz.
- Fisher, Saul.  2013. "Pierre Gassendi" (revised ed.). Stanford Encyclopedia of Philosophy. Retrieved 17 June 2020.
- Kierkegaard, Søren.  1985. Philosophical Fragments, translated by P. Hong.
- Schönbaumsfeld, Genia. A Confusion of the Spheres. Oxford, 2007. p.168-170.
- Archie, Lee C. 2006. "Søren Kierkegaard, 'God's Existence Cannot Be Proved'." In Philosophy of Religion. Lander Philosophy.
- Macmurray, John. 1991. The Self as Agent. Humanity Books. p. 78.
- Descartes, René (1644). Principia Philosophiae. apud Ludovicum Elzevirium. pp. 30, 31.
“Ego Cogito ergo sum”.
- Abraham, W. E. 1974. "Disentangling the Cogito." Mind 83:329.
- Baird, Forrest E., and Walter Kaufmann. 2008. From Plato to Derrida. Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 978-0-13-158591-1.
- Boufoy-Bastick, Z. 2005. "Introducing 'Applicable Knowledge' as a Challenge to the Attainment of Absolute Knowledge." Sophia Journal of Philosophy 8:39–52.
- Christofidou, A. 2013. Self, Reason, and Freedom: A New Light on Descartes' Metaphysics. Routledge.
- Hatfield, G. 2003. Routledge Philosophy Guidebook to Descartes and the Meditations. Routledge. ISBN 0-415-11192-7.
- Kierkegaard, Søren.  1985. Philosophical Fragments. Princeton. ISBN 978-0-691-02036-5.
- —  1985. Concluding Unscientific Postscript. Princeton. ISBN 978-0-691-02081-5.
- Williams, B. 1978. Descartes, The Project of Pure Enquiry. Penguin. OCLC 4025089.