Critique of Practical Reason

Kant purports there to be two basic kinds of action: action motivated by dutifulness and action motivated by self-love. If one acts out of a desire for some particular object, this means that one is acting only under the assumption that he or she has that desire, and so only under the assumption that satisfying that desire will give one pleasure. Self-love is the faculty that pursues one's own pleasure in these cases. If one acts not out of a desire, but rather with the one mere form of lawful willing, one acts out of duty. Pure practical reason is the faculty that produces dutiful action. Moral, dutiful action is motivated by a maxim that is also a law, the categorical imperative. Non-moral action is motivated by the maxim of seeking one's own pleasure, which cannot consistently be held by everyone, and so is therefore not a categorical imperative, or universal law. Non-moral action is not truly free, for one's behavior is determined by the contingent fact of what desires you happen to have. Moral action is the only truly free action. Not only is it free of determinism by contingent desires; it is caused by a conception a person being in the noumenal realm.

Kant recommends that one take advantage of the natural human urge to morally evaluate other people's actions. By challenging students to consider whether a given action is right or wrong, their moral judgment is honed and they are given a chance to feel moral admiration and repulsion appropriately for admirable and repulsive deeds. We must take care to present them with examples in which a person does good purely out of duty, for these will best incite them to reverence and cause them to choose to act in a principled fashion. If we try to mix self-interest and morality, we will not succeed either in making the moral motive clear or in strongly motivating the student. A person who accepts the moral when it is in his best interests is less impressive and inspiring than a person who defies self-interest to follow his moral principles. Nor ought we to rely on examples of people who go above and beyond the call of duty. These examples may emotionally enthuse the student for a time, but will not help him develop a stable commitment to fulfilling everyday obligations.

According to Kant, freedom and determinism are not compatible, although noumenal freedom is compatible with phenomenal determinism. If determinism is true, this means that our current actions follow from the past physical state of the universe. However, since we have no control over the past of the universe, if our actions follow from that, we have no control over them either. One's supposed freedom, even if one's present state depended only on one's past state and not the past state of the universe at large, would be no more than the "freedom" of a mechanical clock which is free to follow its mechanism.

Kant's solution to this is that freedom is to be found in the noumenal world. Every bit of our life is caused by its past, but our self in the noumenal world causes the whole timeline to be as it is. Although the future follows from the past, we have control, noumenally speaking, over the past, and so we can have control over the future.

In Kant's system, practical reason extends further than theoretical reason. So, there are things we are allowed to believe based on the workings of practical reason that theoretical reason says nothing about either way. However, practical reason cannot actually contradict theoretical reason. Nor is it acceptable to believe in anything practical reason takes as an object, but only that which it necessarily does, that is, which pure practical reason takes as an object. In particular, practical reason reveals to us the existence of God and immortality, while theoretical reason says nothing to us about them. Practical reason necessarily aims at the highest good, but for this to be, we must assume God to help bring it about and an eternity in which he can do it.

Kant does not discuss this explicitly in the Critique of Practical Reason, but it seems that he would also take theoretical reason to extend to areas which practical reason does not. For example, practical reason will not have anything to say about astronomy, but theoretical reason will.

Kant's postulates of pure practical reason are theses in which we must believe in order to follow pure practical reason. However, they are also theses about which theoretical reason has nothing to say. The reason Kant gives for believing in God is not an argument for his existence but rather an argument that we would be worse off, morally speaking, if we could not bring ourselves to believe in God. Seeing that there is no evidence to believe in God, as one in Kant's overall position would, one's postulation of God seems in danger of lapsing back into unbelief. On one hand, there is a long tradition of opposing faith to (theoretical) reason, in which one sees faith as an appropriate clinging to belief even against the evidence. On the other hand, the postulates of pure practical reason may seem too calculated. Although Kant claims that the postulates have a different status than self-deception because the needs of pure practical reason are commanding, unlike those of mere desire, we can doubt the relevance of this point.