Editing Cogito, ergo sum (section)

== Interpretation ==
As put succinctly by [[Charles Porterfield Krauth|Krauth]] (1872), "That cannot doubt which does not think, and that cannot think which does not exist. I doubt, I think, I exist."<ref name=Krauth />

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 [[logical truth|necessarily true]] whenever it is put forward by me or conceived in my mind" (''Meditation'' II). [[George Henry Lewes]] says Descartes "has told us that [his objective] was to find a starting point from which to reason—to find an irreversible certainty. And where did he find this? In his own consciousness. Doubt as I may, I cannot doubt of my own existence, because my very doubts reveal to me a something which doubts. You may call this an assumption, if you will; I point out the fact as one above and beyond all logic; which logic can neither prove nor disprove; but which must always remain an irreversible certainty, and as such a fitting basis of philosophy."<ref>{{Cite book |last=Lewes |first=George Henry |url=https://books.google.com/books?id=de8eP3HJIe8C&pg=PA142 |title=The History of Philosophy from Thales to Comte: Modern philosophy |date=1867 |publisher=Longmans, Green, and Company |language=en |quote=See [Descartes's] replies to the third and fifth series of Objections affixed to his Mediations.}}</ref>

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.

{{blockquote|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){{efn|name="abbreviations"|AT refers to Adams and Tannery;<ref name="AT" /> CSM II to Cottingham, Stoothoff, and Murdoch;<ref name="CSM II">{{Cite book|url=https://books.google.com/books?id=Q5P2lFaM8GgC&q=%22The+Philosophical+Writings+of+Descartes%22&pg=PP9|title=The Philosophical Writings of Descartes |volume=II |translator=Cottingham, J. |translator2=Stoothoff, R. |translator3=Kenny, A. |translator4=Murdoch, D. | date=1984 |publisher=Cambridge University Press|isbn=978-0-521-28808-8|language=en}}</ref> CSMK III to Cottingham, Stoothoff, Murdoch, and Kenny<ref name="CSMK III">{{cite book |title=The Philosophical Writings of Descartes |volume=III |translator=Cottingham, J. |translator2=Stoothoff, R. |translator3=Kenny, A. |translator4=Murdoch, D. |year=1991 |publisher=Cambridge University Press |isbn=978-0-521-42350-2 |url=https://books.google.com/books?id=Y3RRKISL810C&q=%22The+Philosophical+Writings+of+Descartes%22+Cottingham,+Stoothoff,+Murdoch,+and+Kenny}}</ref>}}
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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.<ref>''Self, Reason, and Freedom: A New Light on Descartes' Metaphysics'' Andrea Christofidou; chapter 2</ref>  As he puts it:

{{blockquote|[[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){{efn|name="abbreviations"}}}}

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 theory|substance]] with his [[ontology|ontological]] interpretation.