Will and Understanding

Interesting tidbit tonight. Descartes argues that the greatest gifts given to humanity were understanding and will. Understanding is essentially pure and right - we perceive and conceive correctly. I don't interpret this literally, to mean that everybody is always right (although, on an egoist level, this is true), but rather that understanding is not where we fall short. He goes on to reason that the will is where we err - we often take action or pass judgement on that which we do not understand, and this is the source of error. To this point, I agree. The human intellect is surely capable of grappling with the relatively easy mental hurdles we face everyday, especially with the help of others. As Hobbes wrote, this is why all men are created equal - because the dumbest man may outsmart the smartest; the range of human intellect is simply not very big. Where we run into moral dilemmas (and for Descartes, sin) is when we act on that which we do not understand.

I'm not sure where to go from here though. I'm not sure I can accurately judge what I properly or not understand. And this proposition is essentially contradictory - it passes judgement on others who we cannot know for sure have faulty understanding. What if, unbeknownst to Descartes and I, every other person fully understand nature and theology and natural science, and rather chose to act as though they did not? Can this proposition be reasoned out, or must there be an empirical basis for it? And if there is an empiricism present in it... well then how can a man who has a Devil set out to deceive him believe such a thing?