Appendix 45.

(1033) [Descartes' mistaken criterion of certainty]

Descartes was correctly taxed with begging the question when he established his criterion of certainty. He first says:

'Clear perception is the criterion of certainty', and uses this criterion to arrive at the existence of God.

But he then says:

'Clear perception could deceive me, but the existence of God is the reason why the perception cannot deceive me. It comes from God who cannot deceive me'

It seems impossible that his brilliant mind did not see the vicious circle here. But as his error becomes more obvious and necessary, the claim that his system is erroneous acquires greater validity. He knew that the perception of himself as subject needed something else for the perception to be authoritative, because in itself the perception was not necessarily infallible. What was needed was the idea of being, which contains objectivity and necessity. However, because he did not know this truth, he had recourse to the idea of God. He erred therefore in two ways:

1. he argued in a vicious circle because he deduced from perception the very thing necessary to prove perception; and 2. he precluded recourse to common being because he had recourse to the idea of first, subsistent being. The second error set him on the way to his a priori proof of the divine existence. His argument, in the way he presented it, was erroneous: he equivocated by taking the idea of being as subsistent being. However, his efforts and errors prove the necessity of the idea of being (a necessity which I accept), just as much as his authority would have proved this necessity if he had clearly asserted it.