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.
Philosophers since Kant have quarreled with two main problems that arise in this section. First, one might wonder why maxims—the rules that human beings formulate internally when they make choices—have to be either good or bad, rather than both at the same time. Second, one might question Kant's assertion that any action not performed wholly from a sense of duty is evil.
Kant says that maxims cannot encompass both good and bad desires. He believes that every desire that we face, every impulse that competes for our ratification, falls into one of two categories: run-of-the-mill, everyday desires, or the desire to fulfill your duty and do what the moral law requires. He says we can only be good if we do what duty calls for, and when we act on everyday desires and impulses, as we often do, we are acting immorally.
Kant excludes the possibility that maxims can include more than one desire or impulse. Professional philosophers have struggled with this issue, and most of them either admit Kant's belief that maxims are only motivated by one desire, or insist that maxims can, strictly speaking, include more than one desire or inclination. The latter theory appears to be more consistent with Religion within the Boundaries of Mere Reason. For instance, Kant says in 6:24 that free action not based on the moral law must be based upon an everyday desire, and that "it follows that his disposition as regards the moral law is never indifferent (never neither good nor bad)." This quotation shows Kant's idea that an everyday desire and duty can be unified in one maxim, although the resulting behavior must be considered evil, not good.
This brings us to the second problem: why do maxims forged from a combination of duty and everyday desire have to be considered evil? Again, philosophers have given two responses. Some have said that actions done from both duty and desire are not necessarily evil, but rather lack (in Kantian terminology) full moral worth. This response assumes that passages where Kant describes as evil actions motivated by duty and desire are merely exaggerations. Yet some philosophers have said that Kant did mean to call such behavior evil.
Kant might mean to stress that our predisposition to evil is the real problem, not the moral worth of the actions themselves. In 6:30, Kant says that humans have an overwhelming tendency to engage in immoral behavior, and "the mind's attitude is thereby corrupted at its root, and hence the human being is designated as evil".