Descartes knows for certain that he exists because he cannot doubt that he is doubting. He acknowledges himself as a thinking thing that errs from time to time. Descartes looks for the source of error, finding it within two faculties of the mind, the limit of the intellect and the infiniteness of will.
P1: When infinite will meets finite intellect the result is bad judgment.
P2: Bad judgment causes me to err.
C1: I err from time to time.
I will explain the first premise. I will defend the second premise by arguing that bad judgment causes me to err rather than a deceptive nature. Then I will show a sound conclusion.
The will (what I wish) and intellect (ideas I perceive) are not in the same boundaries. We can only know so much. The intellect is limited. The will, however, is not limited. The will extends to things that we do not understand. We can wish the impossible, which strays us from truth and goodness. In that way we can be deceived, therefore passing bad judgment, and err.
For example, with the intellect I am able to perceive a clear, (obvious), idea of brown hair. But maybe what I am actually looking at is black hair under a certain light. My intellect perceives a clear idea of brown hair, but my will passes judgment that what I saw was brown hair, giving me a false sense of certainty (truth). Bad judgment is passed because the infiniteness of will obscures the clarity of a perceived idea. A will that can wish impossible things on what it does not understand will lead an idea astray from certainty (truth), causing bad judgment, and therefore, err, and because possibility for err is limitless then it is possible to err from time to time.
The second premise ‘bad judgment causes me to err.’ Is the weaker premise. A deceptive nature could cause one to err. A nature would be deceptive if God was a deceiver. Once Descartes proved that god was not a deceiver he could be certain that God would not endow one with a deceptive nature. Instead, he has endowed one with a nature that is reliable, if used properly. Therefore, my nature is reliable. If the nature I am giving is reliable then the bad judgment that caused me to err does not come from that nature.
If God is all-powerful why not just make a me that is perfect. (55, pg. 82) Perfect in the sense that I do not err. Maybe God built our nature structurally to err. Descartes asks now whether it should be better to be made in err, rather than in perfection.