For centuries, human beings have been debating over the validity of the use of reason. This is a very, very difficult subject to discuss, as one is forced to study something which is at that moment being used in their study. Two classic thinkers who contrasted on their view of reason were Descartes and Pascal. Though both saw reason as the primary source of knowledge, they disagreed over the competence of human reason. Descartes, the skeptic, said that we could use reason to find certain truth if we used it correctly, while Pascal said that we can't know certain truth, but reason is the best source of knowledge that we have.
Reason is the tool by which we know everything that we know. But most people make the mistake of basing their reasoning on assumptions which are not known with 100% certainty. As I've said, "I am greatly astonished when I consider [the great feebleness of mind] and its proneness to fall [insensibly] into error" (K&B, p. 409). But it is possible to avoid falling into error if we use the valuable tool of reason correctly. In order to do this and find certainty, we must find something that we cannot doubt. This is impossible, as we can logically doubt anything. A certain truth must be something that is not logically possible to be false.
We must doubt, as that is the only way to find certain truth. It is the only way to wipe the slate clean of all of the uncertain assumptions which are believed and taught in the universities today. Just as mathematics will lead to uncertain assumptions if it is not built on certain truths, so will all use of reason lead to uncertain assumptions if it is not built on certain truths. There is a way to use doubt, though, to find certainty. If 100% certainty equals 0% doubt and we are certain that we can doubt everything, then we can use doubt as our certainty. We cannot doubt that we are doubting.
With our one certainty, we can now methodically use reason...