The epistemological principle of Descartes

Det här är min disposition om Descartes som jag ska använda till min presentation om hans princip "Cogito ergo sum" i min muntliga examen i epistemologi, därför är den på engelska.

René Descartes(1596-1650), a french philosopher who was disillusioned with scholastic philosophy. Wanted to achieve certain knowledge based on "clear and distinct ideas". He wanted to overcome scepticism by first accepting it's standpoint in a methodological way, that is he began by doubting everything that could be doubted and making no assumption so that he could find the knowledge that was self-evident, what cannot be doubted. Here he would find his "Archimedean point", on which he would base knowledge that were truly scientific.

So he denies knowledge about the world brought to us by the senses, the knowledge of extension, even the knowledge of mathematics(maybe God or something else is deceiving me into believing that 2 + 2= 4 when it could in reality be 2 + 2= 5, or that things are extended etc).
So everything that he knows can be an illusion, caused by someone deceiving him. But in order for him to be able to be deceived, he has to exist. Something that is being contemplated in his mind might be denied, but not the fact that he is contemplating it. Thus he comes to the one thing that is certain, namely that he exists. Cogito ergo sum(I think, therefore I exist, in the "Meditations" changed to I am, I exist) became the starting point on which he would build his new philosophy, the "upside-down pyramid". From this he could proceed to saying that he is essentially a thinking being(sum res cogitans), which is essentially separated from the body(dualism). For he can think himself without the body, the existence of the body can be denied, therefore it is not an essential part of his thinking being.

But how can he move on? He has proved his own existence, so how can he prove that anything else exists? According to Descartes, we can do this after first proving the existence of God.
We all have the idea of God, and it it the most perfect one we have(also clear and distinct). But surely I can't be the cause of that idea, for I am an imperfect being. Therefore the idea of God must have it's cause from a superior being than ours, that superior being is God(big problems, Descartes assumes "perfection" and "causality", things he has yet to prove). So if God in fact is this perfect being then can we can be certain that He won't deceive us and thus we can trust our abilities to reach truth, our clear and distinct ideas are then really true.
Here comes what is known as the "cartesian circle", Descartes needs God in order to prove the reliability of clear and distinct ideas, but in order to prove God's existence, he assumes the reliability of clear and distinct ideas. In other words, this is what in philosophy is called "begging the question".