In part 4 of his "Discourse on the Method" (titled: "Proof of God and the Soul") René Descartes implements his own method of inquiry described in part 2 and part 3. The important part of this chapter is Descartes' methodological doubt, a maxim according to which anything that can be doubted will be regarded as methodologically false. This will insure to truthfulness of everything that survives this severe scrutiny.
Very much like in his later "Meditations on the First Philosophy", Descartes decides to abandon any sensory information on the charge that those have been known to deceive us. He also abandons everything that might come from experience, holding that we can never know if we are dreaming or not. This pretty much leaves everything we know our but Descartes notes that there is one thing that cannot be doubted and that is that there is someone or something thing which is doubting. Doubt is a thought, and even if that thought is entirely wrong there is still someone performing it. This leads Descartes to his famous conclusion of "I think therefore I am". This assertion will serve as the basis for all knowledge of the world that can now be build upon its certainty. Note that Descartes' existence is only a mental one, at this point of his philosophy the body and anything material for that matter is under doubt and therefore excluded (this is very similar to what Descartes does in meditation 2 and meditation 3 in "Meditations").
After proving that he himself exists, Descartes moves on to ask what else can he be sure of. While doubting objects in reality is very possible, Descartes holds that doubting God in not possible. The reason is that God is perfect, and as such cannot be doubted. Since Descartes is imperfect, it is not possible for him to conceive of something perfect like God on his own (only vice-verse, to perfect can conceive the imperfect). This means not only that God must exist but also that everything he has created must also exist (Descartes later developed his proofs of God in Meditations). After thus establishing his metaphysics in the form of proving the Self and God, Descartes can move on to the physics which is the topic of part 5 and part 6 of "Discourse on the Method".
See additional summaries on Descartes' "Discourse on the Method":
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