For, were it not good that evil things should also exist, the omnipotent God would almost certainly not allow evil to be, since beyond doubt it is just as easy for Him not to allow what He does not will, as for Him to do what He will.
Everyone who observes himself doubting observes a truth, and about that which he observes he is certain; therefore he is certain about a truth. Everyone therefore who doubts whether truth exists has in himself a truth on which not to doubt.... Hence one who can doubt at all ought not to doubt the existence of truth.