In this essay, I propose to explain Descartes's system of methodic doubt. It will be necessary to look at his thoughts throughout his work in "The meditations". I will also show how Descartes subjected to doubt, all that could be possibly doubted, and arrived at the indubitable proposition: Cogito ergo sum, or I think, therefore I am. I will also explain how Descartes proceeded from this basis to prove the existence of God. Finally, I will conclude with how these two propositions together, for Descartes, established the certainty of human knowledge.
Descartes devised his system of methodic doubt in order to discover an indubitable belief, which he could use as a certain, and secure foundation as the basis for knowledge. For Descartes it was necessary to look inside himself at all the beliefs he once held as true, and subject them to the strongest of doubts. For the purpose of freeing himself from all preconceived opinions, he allowed himself to believe that all his past beliefs were false and imaginary, as he states in the first meditation: " So, for the purpose of rejecting all my opinions, it will be enough if I find in each of them at least some reason for doubt." (Cottingham 1984, p12). This provided Descartes with the easiest path by which to lead the mind away from the senses. In considering how far doubt can be extended, he begins by questioning his sense perception.
Descartes realises that everything that he has up till now accepted as true has been acquired either from the senses or through the senses. However there have been times when he has found that his senses can deceive, such as when objects may appear differently from various points of view, and that therefore it is highly probable that other things which appear certain through the senses may in reality be illusions. On further reflection, Descartes ponders the proposition of being asleep or awake. He wonders if this is something that cannot be doubted.