Thr3e, by Ted Dekker

"This is what Kevin could have meant when he told me he had a new model for the natures of man," Dr. Francis said. "The three natures of man. Good, evil, and the man struggling between. 'The good that I would, that I do not, but that which I would not, that I do.' There are really three natures in there! One, the good. Two, that which I would not. And, three, I!"