Descartes,
Rene (1596-1650):
French philosopher and mathematician. Descartes' unique ambition was not to
add a contribution...but to reconstruct the whole of philosophy anew.
"...those who are seeking the strict way of truth should not trouble themselves about any object concerning which they can not have a certainty equal to arithmetical or geometrical demonstration."
A Dictionary Of Philosophy. Second edition. (Pan Books 1984).
Descartes is regarded as the founder of modern philosophy and invented the method of systematic doubt. Whatever it was possible to doubt, he determined to doubt, until he could find good reason for not doubting. He finds it possible to doubt the evidence of the senses, because he makes mistakes about the nature of objects seen in the distance. He can also be mislead by such things as dreams. He even proposes a demon who constantly controls what he experiences to produce an illusory reality of the demon's creating (a virtual reality, something like the film The Matrix). Scepticism is supposedly taken to the ultimate limit in this way.
But Descartes found it impossible to doubt that he existed, because he could think (he could doubt, and the act of doubting is an act of thinking), and this certainty did not rest upon evidence presented by the fallible senses or on any knowledge of anything outside of himself. The very act of thinking, therefore, establishes the existence of the "I". However great the deceptions, the hypothetical demon "can never cause me to be nothing so long as I think I am something... I am, I exist, is necessarily, true as often as I put it forward or conceive of it in my mind." - Second Meditation.
Thus the act of thinking is taken to establish existence, and thus the famous cogito "I think, therefore I am". This is also taken as establishing subjective things as the most certain, seeing as there is nothing we can be more certain of, or know more intimately, than our own thoughts and the "I" we experience every day. We can doubt the world, but we cannot doubt that we can doubt.
At this point in his meditations, the one certainty arrived at so far suggests a division between the mind and body such that the mind certainly exists, whilst the existence of the body cannot be known with such certainty. Descartes claims certain knowledge of what he essentially is: a thinking thing. But if mind is the real "I" then the body may or may not exist. This aspect of his conclusions is known as Cartesian Dualism. Mind and body are separate things which in some way interact.
Having established one certainty, Descartes goes on to attempt to prove the existence of god. Towards this end, Descartes employs a variation of the Cosmological argument (the idea of god is caused by god) and the Ontological argument (the definition of a perfect being involves existence or the being would not be perfect).With the existence of god supposedly established byway of these arguments Descartes goes on to reconstruct the material world his scepticism previously made so uncertain.