In the Critique of Practical Reason, Kant argues that there is one and only one maxim of action suitable to ground morality. This maxim is referred to in his Groundwork for a Metaphysics of Morals as the "categorical imperative", and is best known by that name, although in the Critique of Practical Reason he prefers to refer to it as the Fundamental Law of Pure Practical Reason. The law is that one should "so act that the maxim of your will could always hold at the same time as a principle in a giving of universal law."
Much of the Analytic section of the Critique of Practical Reason is devoted to showing that the categorical imperative is the only possible moral law. It is argued that the law-giving force of the moral law must stem from its mere form—that is, its universalizability—alone, because if it stemmed from the content, the law could only hold for those who cared about that content and not universally.
In the Analytic, Kant argues that freedom and morality are one and the same. The will that is free cannot be acting merely randomly, but must rather be acting on a law. Yet it cannot be dependent on the condition of the sensible world. The only law it can then be following is the law that consists solely of an injunction to follow a lawlike, e.g. universalizable maxim. And that law is just what Kant regards as the moral law. Reciprocally, when one is following the moral will, one is acting independently of one's contingent desires, that is, freely.
Kant presents his view of morality in contrast to what would today be called "compatibilist" theories of freedom, theories that strive to reconcile determinism and freedom. In his eyes, the theory that freedom is being determined by your inner nature, whether or not this is being done deterministically, is comparable to the theory that a clock is free as long as it is following its mechanism. We can see in Kant's view the influence of the Scottish philosopher David Hume. Hume argued that freedom was impossible, for the only two possibilities are that we are determined, in which case we are un- freely following our predetermined sequence of actions, or that we are not determined, in which case we are acting randomly, following chance, which is out of our control. Kant can be seen as proposing a third possibility, a law we can follow which is neither chance nor dependence on the contingent.
Kant stresses that the moral worth of an action is not based on its effects, or on anything else publicly visible about it, but rather on why the agent performed it. Even the person who has just acted may not know what his inner maxim was. It is worth considering whether Kant's moral theory will say anything concrete about what a person should do, as opposed to how he should do things. It has often been pointed out that the same action can be performed with many different maxims.
This might be problematic. If one describes oneself as acting on the maxim of going to a particular café Sunday morning, one cannot universalize that, for there is no room in the café for everyone in the world. Many harmless actions described precisely enough can lead to such problems. Conversely, if I describe a murder in the right way, there will be no problem universalizing it, since we can all will for that one particular person to be murdered without the whole of society collapsing. So it might seem that whether or not I am allowed to perform many actions has only to do with irrelevant features of how I conceive of what I am doing.
Kant opposes his view of morality not only to those who regard the external marks of moral behavior as the most important, but also to those who stress the worth of noble and magnanimous feelings. According to Kant, not only is it unreliable to rely on a person's emotions, which can alter rapidly and without his being able to control them, but also the person who acts morally because of his altruistic feelings is still only acting to please himself, to satisfy his present mood. The truly moral person is the person who acts from a maxim of duty. It is nice and lucky for him and others if he has a kind heart, but whether he is emotionally virtuous or vicious, the important thing is that he holds to performing his duty.
Of course, the unattractive nature of the person who acts dutifully while hating it all the way is obvious, and Kant has often been attacked for this view. One area where Kant's view is especially hard to swallow is that of duty towards one's friends. While we are touched by a person visiting his friend in the hospital due to his concern for her, we feel far from enthused about the person who lacking such care comes to see his friend out of a feeling of duty.
It is true that we prefer the person who simulates a good character out of a sense of duty to someone who simply luxuriates in his viciousness, yet is this, the dutiful person, the best model for moral behavior in general? One might be more inclined to think of acting purely for duty as what the good person does on the relatively rare occasions when he cannot connect emotionally to his situation in the right way.
In the Analytic of the Critique of Practical Reason, Kant provides us with reason to believe in one noumenal object, our freedom—when we feel the moral law on us, we feel our freedom to obey it. In the Dialectic, we are given reason to believe in two more noumenal objects, God and immortality.
The aim of the moral will is the highest good. Although this is true, because the highest good is not to be found in this world, it is confused to say that that is where we must aim. The highest good requires both our moral perfection and our well-being proportionate to our moral perfection, but we are not capable of bringing about either of those. Yet we could not in good will follow the moral law unless we believed that somehow or other the highest good would follow from it.
It is God, according to Kant, who will bring about our ultimate happiness commensurable to goodness. He will bring it about in the afterlife, which we would need to believe in anyways, since only in an eternal afterlife can flawed humans reach moral perfection.