Cartesian circle

The Cartesian circle is a mistake in reasoning allegedly made by René Descartes.

Descartes argues – for example, in the third of his Meditations on First Philosophy – that whatever one clearly and distinctly perceives is true (that is, logically possible): "I now seem to be able to lay it down as a general rule that whatever I perceive very clearly and distinctly is true." (AT VII 35) He goes on in the same Meditation to argue for the existence of a benevolent god, in order to defeat his sceptical argument in the first Meditation from the possibility that god be a deceiver. He then says that, without his knowledge of god's existence, none of his knowledge could be certain.

Descartes' contemporaries

Many commentators, both at the time that Descartes wrote and since, have argued that this involves a vicious circle, as he relies upon the principle of clarity and distinctness to argue for the existence of god, and then claims that god is the guarantor of his clear and distinct ideas. The first person to raise this criticism was Marin Mersenne, in the "Second Set of Objections" to the Meditations:

"you are not yet certain of the existence of God, and you say that you are not certain of anything, and cannot know anything clearly and distinctly until you have achieved clear and certain knowledge of the existence of God. It follows from this that you do not yet clearly and distinctly know that you are a thinking thing, since, on your own admission, that knowledge depends on the clear knowledge of an existing God; and this you have not proved in the passage where you draw the conclusion that you clearly know what you are." (AT VI 124–125)

Descartes' own response to this criticism, in his "Author's Replies to the Second Set of Objections", is first to give what has become known as the Memory response; he points out that in the third Meditation (at AT VII 69–70) he didn't say that he needed god to guarantee the truth of his clear and distinct ideas, only to guarantee his memory:

"when I said that we can know nothing for certain until we are aware that God exists, I expressly declared that I was speaking only of knowledge of those conclusions which can be recalled when we are no longer attending to the arguments by means of which we deduced them." (AT VII 140)

Secondly, he explicitly denies that the cogito is an inference: "When someone says 'I am thinking, therefore I am, or I exist' he does not deduce existence from thought by means of a syllogism, but recognizes it as something self-evident by a simple intuition of the mind." (AT VII 140) Finally, he points out that the certainty of clear and distinct ideas doesn't depend upon god's guarantee; the cogito in particular is self-verifying, indubitable, immune to the strongest doubt.

Modern commentators

Bernard Williams presents the memory defence as follows: "When one is actually intuiting a given proposition, no doubt can be entertained. So any doubt there can be must be entertained when one is not intuiting the proposition." (p.206) He goes on to argue: "The trouble with Descartes's system is not that it is circular; nor that there is an illegitimate relation between the proofs of god and the clear and distinct perceptions [...] The trouble is that the proofs of God are invalid and do not convince even when they are supposedly being intuited. (p. 210)

As Andrea Christofidou explains:

"The distinction appropriate here is that between cognitio and scientia; both are true and cannot be contradicted, but the latter is objectively true and certain (with the guarantee of God), while the former is subjectively true and certain, that is, time-bound, and objectively possible (and does not need the guarantee of God)." (pp 219–220)

Sources and references

• René Descartes, The Philosophical Writings of Descartes Volume II, translated John Cottingham, Robert Stoothoff, and Dugald Murdoch (Cambridge University Press, 1984) ISBN 0-521-28808-8
• Andrea Christofidou, "Descartes' Dualism: Correcting Some Misconceptions" (Journal of the History of Philosophy XXXIX:2, April 2001)
• Bernard Williams, Descartes: The Project of Pure Enquiry (Penguin Books, 1978) ISBN 0-14-022006-2