Kant is famous for his skeptical view of human nature. In some passages he seems to suggest that human nature is unqualifiedly evil, and that only a daily regimen of moral deliberation can quash our natural badness. In others he says that along with our inclination toward evil, we all possess a germ of good. He believes that humans can be either good or evil, not both at once. We use general rules called maxims to make decisions; all actions not based on maxims are simply responses to impulses and desires. This tendency to make decisions without thinking of our maxims is one of the signs of our evil natures. Kant calls it the frailty of human nature.

In addition to falling short of our ideals, our good behavior sometimes stems from selfish considerations. We do not always undertake the moral action for its own sake, but because the moral action also happens to save us money, or win us attention, or benefit us in some unseemly way. This tendency to augment a sense of duty with immoral incentives is the second variety of our inclination to evil, which Kant calls the impurity of human nature.

There is a third variety of the inclination. Kant calls it the depravity of human nature. Depraved people consciously allow immoral incentives to drive their behavior.