Meditations - xlii

It is only now, having demonstrated the existence of God, that we finally liberated from uncertainty and doubt. Knowing that God exists and that we are created by Him, we can both explain the presence in our souls of clear and distinct innate ideas, and justify our assurance of their validity: it is God, indeed, who endowed us with them; it is God, therefore, who guarantees their truth, that is, their conformity with the real world created by Him. God's veracity is thus the ultimate foundation of our reasoning, of the right that we have to conclude from the idea to the thing which it represents, to assert, for instance, the real existence of extension and motion, the validity of the mathematical sciences and of the physics based upon them. The reasoned-out confidence that we have in our reason is thus, for Descartes, justified only and alone by the reasoned-out confidence that we have in God. An atheist, denying the existence of God, must, therefore, necessarily be the prey of an absolute scepticism: he cannot have an assurance of anything whatever -- not even of mathematics -- and, for him, to believe his reason would be utterly unreasonable.