Descartes revitalized philosophy by his rejection of the Aristotelian and Scholastic traditions of the "schoolmen".
Descartes, in Discourse on Method and Meditation, claimed to have proved the existence of God.
In brief, Descartes’ argument runs as follows. Descartes first claimed* to have discarded all received information:
• My senses may mislead me, and I may have been deceived by a malignant demon.
So, I can only be certain of the content of my consciousness, and thus know that I exist as a thinking being – cogito ergo sum.
• I comprehend the nature of corporeal objects – a ball of wax – more fully with my intellect than with my senses. Concerning my clear and distinct ideas, I could have no assurance of the truth of my perceptions if any proved to be false.
So, as a general rule all that is clearly and distinctly apprehended (conceived) is true.
• I could be deceived about the truths of arithmetic and geometric objects, but my idea of God does not include His being a deceiver.
So, God is not a deceiver.
• Assuming that ideas conform to external things is the commonest error of judgment. Some such ideas are innate, others adventitious, and others factitious. Considering ideas as images, I note great degrees of diversity. Ideas that represent substances have more objective reality (content concerning finite substances) than those that represent modes or accidents. The idea by which I conceive a God has more objective reality than those ideas by which finite substances are represented.
Conclusion: God exists. God exists according to the preceding arguments, and because: It is manifest by the natural light [innate wisdom] that there must be as much reality in cause as effect, both for real objects and for ideas with objective reality. My imperfect, finite self could not have originated the idea of a perfect, infinite God, nor could the idea of God have come from nothing. My idea of the infinite is a true idea and not a negation of the finite.
• I did not bring myself into existence because I am not God and do not possess God’s perfection. A different God could not have created me because only one such God can exist according to my concept of God. My parents did not produce me as a thinking being, they do not conserve me from one of an infinity of independent instants into the next. God did not create me in his own image (counter to atheist claims), and I cannot complain that God allows me to be mistaken.
Conclusion: I could not have come to exist, nor be conserved in that existence except by the agency of God.
The most obvious difficulty with Descartes’ argument lies in its fatal circularity. However, problems with unacceptability of premises strike the reader before Descartes begs the question in his conclusion.
One of the most glaring errors in Descartes' argument lies in the following premise, "My imperfect, finite self could not have originated the idea of a perfect, infinite God, nor could the idea of God have come from nothing. My idea of the infinite is a true idea and not a negation of the finite."
The idea of a perfect, infinite God predates Descartes by thousands of years since the Christian God is actually the Jehovah (Yahweh) of Judaism. So, pious, Jesuit-educated Descartes clearly must have obtained this idea through Christian teachings. He conveniently*, and dishonestly, ignored this obvious fact for the sake of his highly contrived and ultimately ineffectual argument.
Descartes quickly and unknowingly refutes his own argument when he states, “But if I desire to think of a chiliagon, I indeed rightly conceive that it is a figure of a thousand sides, as easily as I conceive that a triangle is a figure composed of only three sides; but I cannot imagine the thousand sides of a chiliagon as I do the three sides of of a triangle, nor, so to speak, view them as present [with the eyes of my mind].”
In essence, Descartes tries–and fails–to prove that the subject of an idea has reality outside the idea.
* Descartes failed to discard the inculcated notion of God. To do so would obviously have defeated the purpose of his argument. Considering the religio-political climate in which Descartes lived, his bias is understandable.