Repost: Part 4 of Descartes' Discourse

This is the part of the discourse in which Descartes explains how he went about doing the meditations. Descartes decides to search for truth which is indubitable- something he could be completely certain of. He uses a method of methodological doubt to do this. He begins by doubting everything that he cannot be completely certain of, i.e. physical objects and the external world in general. This kind of doubt is methodological instead of pathological because it is not as though Descartes is a crazy man who leaps to the conclusion that he cannot know anything. Rather, he wants to call everything into doubt that he cannot be certain of, so that out of all of this he can sift something which he can be certain of. He says that almost the entire physical world which he experiences could plausibly be just a dream, seeing as dreams seem just as real as reality while we are sleeping. He comes to one thing which he cannot doubt: his own existence. He knows this for sure because of the proposition "Cogito ergo sum" ("I think therefore I am"). If he is thinking, then it must be that he exists or else he would not think, and the instant that he doubts this proposition he affirms its truth, because to doubt is to think. He concludes that if anything is certainly true, this is.