Religion within the Boundaries of Mere Reason is organized into four parts, but for our purposes these four large divisions will be broken up into ten smaller, more manageable sections. In the present section, Kant explores two main issues. First, he explores how it is that we know human nature to be either inherently good or inherently evil. Secondly, he explains that although humans have a natural propensity to do what is right, this tendency is consistently overshadowed by a propensity to engage in morally corrupt behavior.
Let us begin with the first question: how do we know, with certainty, that human nature is either inherently evil or inherently good? After all, human nature is a complicated thing, and perhaps it is not only evil or only good. It seems reasonable to think that human nature is partly good and partly evil.
Kant rejects the theory that human nature is a mix of good and evil, inviting us to consider the following argument: every time a human being acts freely—that is, acts of his or her own free will—a law or a general rule is formulated inside the person. Kant calls this law a maxim. A maxim's primary function is to ensure that impulses do not directly dictate our behavior. The Golden Rule is a maxim, for instance, albeit an abstract and general one. But why do we need maxims? Why not allow our impulses and desires to guide our behavior directly? Kant says that in order to act freely, we must have some power to ratify or reject our desires. If our desires overwhelm us and we have no veto-power, then we cannot say that we truly act freely. Maxims allow us to accept or reject a given desire, and hence allow us to act freely. Because a maxim is good only duty inspires it, human nature can only be good (in accordance with duty) or evil (in accordance with everyday desires).
The previous argument only addresses what happens in particular instances of decision making, which does not directly prove that human beings are either good or evil by nature. To draw the conclusion that human beings are either good or evil, Kant has to show that we typically ignore duty, instead choosing to act on our everyday desires. In other words, Kant's argument only becomes complete when he explains why we are, by nature, consistently influenced by evil desires and impulses.
The second question in this section asks, if we have a tendency to do what is right, how is it that we are consistently swayed by evil desires and impulses? Kant claims that our propensity to do what is right comes in three forms: the propensity to preserve our own species (survival), the propensity to seek the respect and affection of others (social needs), and the propensity to regard the moral law as important enough to follow consistently. Kant acknowledges and rejects that theory that survival and social needs sometimes conflict with the demands of the moral law.
Kant believes that our moral constitution is weak in three distinct ways. First, we are frail, which means that often we do not act in ways that we know to be morally upright. Second, we are impure, which means that we sometimes act morally only when doing so also suits our interests. Finally, we are depraved, which means that often we act in direct opposition to what we know to be right. In each of these cases, our moral constitution freely chooses the immoral alternative. In Kant's view, we do not choose badly because someone forcues us, or because our physical and psychological needs require it, but because we consciously choose to ignore what we know to be morally right.