In his six Meditations, only four contain his argument about corporeal things and establishing himself as a thinking thing. Meditations three and four discuss the existence of God and the matter of true and false. Concerning Meditation three, Descartes proves God's existence and that He is not a deceiver, thereby allowing us to be sure that we are not deceived when we perceive things clearly and distinctly. The rest of the Meditations deal with him proving himself as a thinking thing and proving that the mind is separate from the body. In Descartes' first meditation, he goes on to prove that nothing exists. He establishes that knowledge is built upon a foundation; each piece of knowledge rests upon some other part of knowledge. Over the course of ones life, a person establishes one piece of knowledge and builds upon that. Descartes goes on to doubt every particular set of knowledge he has.
Descartes says that the most basic set of knowledge we have is our senses. He continues that the senses give us false information. For example, when we look at the sun, we cannot tell how big it is. The same is true for dreams. Senses appear to be real in dreams, but how can one tell whether or not we are dreaming or not. So if we can never determine we are dreaming or awake then we can't rely on our senses.
He believes that a supreme God has created us and has the power to deceive us. If God is perfect then he cannot deceive us. So we must assume that an evil demon is the source of our deceptions. Therefore Descartes has reason to deny the validity of his senses.
From this, Descartes assumes if there is a deceiver and he can be deceived then he must exist. In general it will follow from any state of thinking, whether it be imagining, sensing, feeling, or reasoning, that he exists. Since he can only be certain of the existence of himself insofar as he is a thinking thing, then he has knowledge of his existence of only a thinking thing.
After he has established himself as a thinking thing, he then goes on to argue that the mind is more certainly known then the body. He goes on to say that it is possible that all knowledge of external objects, including his body, could be false as the result of the actions of an evil demon. It is not, however, possible that he could be deceived about his existence or his nature as a thinking thing. This is true because if he can be deceived about anything, then he can be certain, as he is a thinking thing. Even corporeal objects, such as his body, are known much more distinctly through the mind than through the body.
This is where the wax argument comes into play. All the properties of the piece of wax that we perceive with the senses change as the wax melts. This is true as well of its primary properties, such as shape, extension and size. Yet the wax remains the same piece of wax as it melts. We know the wax through our mind and judgement, not through our senses or imagination. Therefore, every act of clear and distinct knowledge of corporeal matter also provides even more certain evidence for the existence of Descartes as a thinking thing. Therefore his mind is much clearer and more distinctly know to him than his body. At this point in Descartes' Meditations he has first distinguished the mind from the body.
Since he proved that God does exist he can says...