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In Meditation III, Descartes puts forward the following general rule, the Truth Rule :
(TR) “So I am now seem to be able to lay it does as a general rule that whatever I perceive clearly and distinctly is true.”
Descartes then says that he clearly and distinctly perceives that certain things, such as his own existence and simple mathematical truths. The only reason he could find for not trusting these clear and distinct perceptions is the hypothesis of a deceiving God. In order to be sure, therefore, that he really can trust his clear and distinct perceptions, he needs to remove this “slight and metaphysical doubt”. He must establish (i) that there is a God and
(ii) that he is no deceiver: without knowledge of the two truths that God exists and that God is not a deceiver, he cannot be certain of anything. At the end of Meditation V, Descartes adds by way of reinforcement:
“Thus I see plainly that the certainty and truth of all knowledge depends uniquely on my awareness of the true God, to such an extent that I was incapable of perfect knowledge about anything until I became aware of him.”
Arnaud, his fourth commentator, remarks.
“I have one further worry, namely how the author avoids reasoning a circle when he says that we are sure that what we clearly and distinctly perceive is true only because God is or exists . For we can only be sure that God exists because we conceive it clearly and most distinctly; therefore, before being assured of the existence of God, we must be assured that all the things that we conceive clearly and distinctly are all true.” [MMO IV::CSM II:150]
This is the famous Cartesian Circle. In order to know that my clear and distinct perceptions are true, I need to prove that God exists. But to prove that God exists, I’ll need some true clear and distinct perceptions to build a proof. I’m moving in a circle. Can I break out of it?
Let’s represent the problem as follows:
(TR) I clearly and distinctly perceive that p

p is true
(GOD) God exists and is a non-deceiver.
(1) I can know (TR) only if I first know (GOD)
(2) I can know (GOD) only if I first know (TR)
Descartes wants to prove (TR) and (GOD) but seems to say both (1) and (2). This is not permissible. Either you take (1) and ignore (2), establish (GOD) and then conclude (TR) or you take (2) and ignore (1), establish (TR) and then conclude (GOD). So, you’ve got to deny one of (1) or (2).
Descartes was not troubled by the accusation. He refers Arnaud to what he said in his replies to the second set of objections.
“Where I said that we cannot know anything for certain if we do not first know that God exists , I said quite deliberately that I was talking of knowledge ( scientia ) of such things, the memory of which may come back to us when we are no longer thinking about the arguments from which we have deduced these things.” [MMR II::CSM II:100] 1
1 See also MMR IV::CSM II 171: “To begin with, we are sure that God exists because we attend to the arguments that prove this; but subsequently, it is clear enough for us to remember that we perceived
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In order to understand this properly, we need to revisit the idea of a clear and distinct perception. Descartes spoke of the lux naturae or the “natural light of reason” in whose illumination we could have clear and distinct perceptions, of which he says:
“I call a perception “clear” when it is present and accessible to the attentive mind – just as we say that we see something clearly when it is present to the eye’s gaze and stimulates it with a sufficient degree of strength and accessibility. I call a perception “distinct” if, as well as being clear, it is so sharply separated from all other perceptions that it contains with itself only what is clear.” [PP I 45::CSM I:207-8]
The key point about a clear and distinct perception is that it is a mental act of keeping a thought in focus and so when one no longer has it before the mind’s eye, one cannot be certain whether it is true or not. It is rather like being able to see clearly the colour of a wall before your eyes but being unable to bring back to mind the colour when you turn away with such a degree of conviction that you are convinced that you have imagined the same colour.
What Descartes is after is truth but not merely truth in of transitory or fleeting nature that only is manifest when the light of reason is upon it but a repository of truths whose status is secure when they are not being explicitly attended to. This is scientia or scientific knowledge. God’s role is to be the guarantor of the veridicality of our scientific thoughts. An atheist can clearly and distinctly perceive that 2+3=5 but not incorporate it into a body of knowledge.
This suggests a strategy called the Memory Strategy:
(MS) Deny (1). (TR) expresses a basic fact about how we can perceive the truth.
But it limits our perception of the truth to the here-and-now. So we need to prove
(GOD) so that when we’re not considering a proof of p but remembering that we have proved p , we can know our memorie s don’t deceive us because God isn’t a deceiver and wouldn’t have allowed us to have such fallible memories.
Descartes does appear to talk as if this is what he wants to say in some places but not in others. Descartes does seem to be pretty clear that (TR) needs the backing of God in
Meditation III. In any case, the strategy is not promising. Firstly, I would have to be able to prove God’s existence in a single act of thought and that seems impossible. Secondly, suppose I (i) prove God’s existence and then (ii) reason that my memory that I did q yesterday must therefore be true. When I do (ii), must I not also be relying on the memory that I have proved God’s existence? Indeed, but I just did, and, given that I can therefore trust my memories, I can rely on my memory that I proved that God exists.
But this won’t work. The moment my thinking of the proof falls into memory, it becomes useless. I have memory of proving that God exists – why trust it? Because you did indeed prove that God exists. But that’s only so if I can trust the memory! I’m reasoning in a circle again.
Philosophers are pretty much agreed that the Memory Strategy is not what he had in mind.
They do not however agree on what he did have in mind. something clearly in order for us to be certain that it is true. This would not be sufficient if we did not know that
God exists and is not a deceiver.”
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