Rene Descartes wanted to eliminate belief in all things about which he is not certain, so as to place philosophy on a foundation of solidity equal to that of modern science.
His path to this solidity of philosophy is his system of philosophical skepticism (i.e. methodical doubt). He will negate all that he is not certain of being true. He proceeds to determine that all that he had previously believed rests on one foundation. For Descartes, the set of knowable statements is to be exactly equivalent to the set of statements about which he is certain of being true.
Doubt is Descartes' criterion for disbelief. We have here philosophical skepticism. Descartes authors an epistemology of certainty. That which is not certain is not known. The slightest notion of doubt that he has about the truth of any statement will cause him to disregard the statement as true. His own expression of this methodical doubt is:
"I must avoid believing things which are not entirely certain and indubitable" (Meditations, 95).
Thus, Descartes is an a position to eliminate all of his false beliefs with one drop of the axe. He says:
"Everything I have accepted up to now as being absolutely true and assured, I have learned from or through the senses" (95).
In a move that anticipates Hume's later acceptance of scepticism, Descartes decides that the
senses (i.e.,
empirical knowledge) are the foundation of all the
untruth that he previously held as knowledge. He proceeds to examine the senses and sensory knowledge and concludes that there is no empirical knowledge. In this way Descartes claims to have undermined everything he had previously believed to be true.
That Descartes' solution to the philosophical problem of sensory knowledge is not satisfactory has been argued by a number of philosophers, though just as many have probably argued the reverse. Descartes' methodical doubt, hand-in-hand with his metaphysical dualism and his insistence on the primacy of epistemology in philosophical and critical activity is certainly a key starting point for modern philosophy as we know it.