In the Meditations, Descartes tests his ideas – for example, his idea that material objects exist – by examining the reliability of his sources (his senses, his nature, the author of his being). His view seems to be: I cannot be sure of anything unless I can be sure that my maker is perfect.
In efforts to prove things such as truth, God's existence, and that material objects exist, Descartes was trying to give philosophy the same certainty that he saw in mathematics. He would attempt these proofs with rational arguments which were based on this idea of the firm foundation of mathematics.
In the first meditation, Descartes doubts everything, including material objects. Every thing seen and sensed could be a dream (the Dream Argument), a hallucination, or an illusion provided by a superior consciousness/being (The Evil Demon Argument).
In the second meditation, he determines that the first certainty is that he is a thing that thinks. He then proceeds to determine other things with this same kind of certainty. In his "Wax Argument," Descartes claims that since the color, shape, and smell of the wax disappears when the wax melts, those qualities must be intuitions/sense impressions of the mind. Therefore, as those are created and/or disappear, there must be some element that causes these intuitions to appear and disappear. The element is something that affects the mind/intuitions and the element; something other than the thinking thing (mind) must be outside of it and therefore an object in the material world. Here, Descartes establishes a separation of mind and matter.
He will use his proof of his own existence (mind as thinking thing and immaterial) to prove the existence of the external material world. He reasons that even if he is deceived - that the external world is an illusion - he is still a thing that thinks. Although still dubious, he rests with the idea that external materials must somehow affect the mind (as the body affects the mind). This Cartesian mind/body dualism parallels the arguments of the mind/external world.
In the third meditation, Descartes has the idea of a perfect God. He maintains that this idea (any idea) must be caused by something/someone. He can not come up with this idea himself because he is not perfect. Therefore, a God (infinite and perfect) must be the cause of his idea of an infinite and perfect God. God has given this idea of an infinite and perfect God to Descartes innately. Thus, God exists based on His (God's) being the cause of the idea. An infinite and perfect God must be benevolent/loving, therefore such a God would not be a deceiver. Therefore, I (Descartes - the mind's "I") am caused by God and would not be deceived and therefore the external world is as real as I AM (as I think). Descartes' reason is that a benevolent God would not deceive him; therefore, the material world exists. He is not as specific as to how his mind interacts with the material world.
Descartes is as certain that God exists as he is that he exists and that a triangle exists. This, again, goes to the thinking that even if the triangle is only an idea in his mind, it must have been caused by something. If if was not caused by him, it must BE, or have been caused from, elsewhere. Descartes goes this same route with proving the existence of God, and he solidifies his belief in God and the external with the reasoning that a perfect, benevolent God would not deceive him.
Paper One (draft two)
Paper One (draft two)