Descartes’ cogito and certainty

By Tim Harding

Rene Descartes (1596-1650CE) was a French mathematician, scientist and philosopher.  According to Copleston (1994:63-89) these three interests of his were interrelated, in the sense that he had a mathematical and scientific approach to his philosophy.  Mathematics ‘delighted him because of its certainty and clarity’ (Copleston 1994: 64).  His fundamental aim was to attain philosophical truth by the use of reason and scientific methods.  For him, the only kind of knowledge was that of which he could be certain.  His ideal of philosophy was to discover hitherto uncertain truths implied by more fundamental certain truths, in a similar manner to mathematical proofs (Copleston 1994: 66-70).

Using this approach, Descartes (1996) engages in a series of meditations to find a foundational truth of which he could be certain, and then to build on that foundation a body of implied knowledge of which he could also be certain.  He does this in a methodical way in his First Meditation by first withholding assent from opinions which are not completely certain, that is, where there is at least some reason for doubt, such as those acquired from the senses (Descartes 1996: 12).


Next, in his Second Meditation, Descartes concludes that one proposition of which he can be certain is ‘I am, I exist’ (Descartes 1996: 12).  Interestingly, in this text Descartes does not actually use the famous words ‘Cogito, ergo sum’ (which mean ‘I think, therefore I exist’) which he used in a slightly earlier work Discourse on Method.  This difference in wording has implications for the discussion which follows in this essay; however, for simplicity, I shall refer to this proposition as ‘the cogito’.

The central question for this essay is – how did Descartes come to be certain that the cogito is true?  There are rival interpretations of the basis of this certainty.  Is it as a result of an inference from the premise ‘I think’, or is it derived from a different type of reasoning in which ‘I think’ is not needed as a premise?  The former is in the traditional form of an argument in which a conclusion is logically deduced from one or more premises; whereas the latter is in not in the form an argument, such as intuition, or as I shall later suggest a ‘performative utterance’.

One of the difficulties in making these interpretations is that Descartes himself is not entirely consistent in the various expositions of his views in different texts, nor in his responses to objections to those views.  Another difficulty is that certain philosophical or linguistic concepts such ‘performative utterance’ had not been developed that at that time.  To clarify, I intend to analyse the cogito in terms of modern day philosophy, rather than as a historical investigation into what Descartes meant at the time.

The first interpretation is that the cogito is a deductive argument with a missing but implied first premise in the following traditional syllogistic form:

Premise 1: Everything that thinks exists.

Premise 2: I think.

Conclusion: Therefore, I exist.

This is a valid deductive argument known from antiquity as modus ponens.  The general form is ‘P implies Q; P is asserted to be true, so therefore Q must be true’.  As is the case with all valid deductive arguments, if the premises are true, then the conclusion must be true by virtue of the argument’s logical form.  However, the problem in this particular case is that we do not know that Premise 1 is true – that has not yet been established.  So although the argument is valid we cannot say that the conclusion ‘I exist’ is true on this basis.  For this reason, I do not think that this interpretation of the cogito is the correct one.

It is worth mentioning that Descartes himself denies that the cogito is a syllogism in his reply to the Second Objections:

When we observe that we are thinking beings, this is a sort of primary notion, which is not the conclusion of any syllogism; and, moreover, when somebody says ‘I am thinking, therefore I am or exist’, he is not using a syllogism to deduce his existence from his thought, but recognising this as something self-evident, in a simple mental intuition (Descartes 1996: 68).

Descartes’ words in this quotation are consistent with the alternative interpretation that the proposition ‘I exist’ is self-evident as a result of intuition.  In this interpretation, we do not need either of the premises ‘Everything that thinks exists’ or ‘I think’ so there is no inference or deductive argument, let alone a syllogism.

I find the notion of intuition too vague for philosophical purposes – it seems to belong more in the realm of psychology or neuroscience.

Williams (1978) has endeavored to explain this alternative interpretation in terms of incorrigibility and self-verification.  A proposition p is incorrigible when it satisfies this description: if I believe that p, then p, for example ‘If I feel pain, I am in pain’.  This explanation has some similarities to Austin’s concept of a performative utterance (or ‘performative’ for short) where the utterance of a statement (in the appropriate circumstances) serves not only to describe an act but to actually perform the act (Austin 1962: 6).  So to say ‘I exist’ performs the act of existing.  The statement could not be made unless the person making it exists.

According to Williams (1978) the proposition ‘I think’ is self-evident because it satisfies the description: if p, then I believe that p.  If I think, then I believe that I think.  The proposition ‘I think’ is thus evident to me, in a way that the proposition ‘I exist’ is not.  While ‘I exist’ is incorrigible, it is not evident to me in the same way that ‘I think’ is evident.  Under this interpretation, Descartes has a reason for choosing to begin his argument with the premise ‘I think’ (Townsend 2004: 26).

In summary, the interpretation of the cogito being an inference or deductive fails for the reasons I have given.  In my view, the combination of incorrigibility and self-verification provides a sufficient justification for the truth of the statement ‘I think, I am’, especially when incorrigibility is explained in terms of a performance utterance.


Austin, J.L. (1962) How To Do Things With Words. London, Oxford University Press.

Copleston, F. (1994) A History of Philosophy Volume IV: Modern Philosophy. New York, Bantam Doubleday Publishing Group.

Descartes, R. (1996) Meditations on First Philosophy: With Selections from the Objections and Replies, trans. and ed. John Cottingham, Cambridge, Cambridge University Press.

Williams, B. (1978) ‘Descartes: the Project of Pure Enquiry’ in Descartes and the Defence of Reason, 2004Study Guide ed. Aubrey Townsend, Clayton, Monash University.

Townsend, A. ed. (2004) Descartes and the Defence of Reason, Study Guide, Clayton, Monash University.

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