Almost since Descartes published his book Meditations he has been accused of committing a fallacy of circular reasoning with his argument that God is the guarantor of the truth of our belief in an external world. Descartes’ argument has ever since been derided as the “Cartesian Circle.” The argument against the so-called “Cartesian Circle” is actually a fallacious strawman argument that misrepresents what Descartes actually argues.
The accusation against Descartes is that he asserts that the existence of God verifies that ideas that are clear and distinct must be true. So those who argue the “Cartesian Circle” position are claiming that Descartes is arguing the following:
1. I have a clear and distinct idea of God as a perfect being.
2. God, a perfect being, is not a deceiver and would not allow me to be mistaken about my clear and distinct ideas.
3. Therefore, I can be certain of the truth of my clear and distinct ideas.
As the Encyclopedia Britannica states it (I think a reasonable summation of the standard interpretation of the Cartesian Circle):
But Descartes cannot know that this proof does not contain an error unless he assumes that his clear and distinct perception of the steps of his reasoning guarantees that the proof is correct. Thus the criterion of clear and distinct perception depends on the assumption that God exists, which in turn depends on the criterion of clear and distinct perception.
This is a valid assessment of the argument stated above. The question though is whether that is an accurate portrayal of Descartes’ actual argument.
Garret Thomson in Bacon to Kant argues that it is not. The key to understanding Descartes’ argument is to discern the difference between particular experiences and the whole of experience.
Descartes uses the proposition “Ideas that are clear and distinct must be true” as his foundational principle. All else follows from that. The proposition is true independent of God. Any idea that can be said to be clear and distinct then must be accepted as true. The existence of God is not a “that X” proposition, it is a clear and distinct idea, God cannot be doubted. The existence of God does not support the proposition “Ideas that are clear and distinct must be true” it guarantees that our sensory perception is trustworthy, or as Thomson puts it on p. 35, “God is introduced to meet our general systematic doubts.” We can, as Descartes points out, doubt that the external world exists; we could be deceived. God ensures that we are not deceived because God is not a deceiver.
So, Descartes’ reasoning goes like this:
1. Ideas that are clear and distinct must be true
2. I have a clear and distinct idea of God
3. Therefore, God must exist.
4. Since my idea of God includes his perfection, God is trustworthy
5. Therefore, I cannot be deceived about the existence of the external world because God would not allow me to be deceived.
Descartes is not saying that he cannot be mistaken about particular propositions such as “that X” because he knows his will can overreach and lead him to error. But because he knows he is a mind and because he knows God would not let him be deceived, then knowledge from experience is possible. Thus, if he can reason correctly (his four rules of scientific method) then his mind is incapable of error because he can arrive at clear and distinct ideas.
Descartes’s reasoning can be questioned in several of its premises, but it is not circular reasoning. The accusation of the “Cartesian Circle” is a fallacious strawman argument.