Rene Descartes lived from 1596-1650; he was a rationalist who wrote a book named, Meditations on First Philosophy. Descartes challenges the idea of certainty. Descartes presents the idea of an Evil Genius. The Evil Genius is a being that implants what we think, our perceptions, our ideas, and our thoughts in our mind that distract us from what is really the truth. The Evil Genius is incapable of love or goodness, which makes the Evil Genius imperfect. .
Since the Evil Genius is in control of what we think, Descartes is left questioning what is certain. Descartes explains that nothing is certain because the Evil Genius could just implant anything we think that is true or any idea that we have into us; this makes absolute truth impossible. By setting this idea up, Descartes broke everything down that is certain. Then, Descartes built everything back up by there being a certainty of God. He was able to establish that there was a certainty of God by laying a foundation to build on which was an idea. He first said an idea must have a cause, next he said that the cause must be as real as the effect of the cause. Through this foundation, he attempted to prove the existence of God.
In his first argument, he was able to draw to the conclusion that there is a certainty of God through the idea of imperfection. Descartes thought that if there was an idea of imperfection, then that implies that there must be an idea prior to that of what perfection is. He thought that the idea of perfection was positive and that it either came from himself, someone else, other beings, or an infinite God. Through reasoning, Descartes ruled out that that himself, other people, and beings are not perfect and what's left is that God is the perfect being. Thus God exists. Descartes drew his next conclusion that God exists through the idea of perfection. Descartes thought that if you believe in God, then believing entailed the characteristic of God that was being infinite.