The Critique of Practical Reason contains two sections, the Doctrine of Elements, containing the Analytic of Pure Practical Reason and the Dialectic of Pure Practical Reason. These section headings are the same as those of the Critique of Pure Reason. Overall, the Analytic contains the arguments for the categorical imperative as the one true moral principle and for the identity of morality and freedom, the Dialectic exposes the primary error of all previous ethicists and proposes the postulates of pure practical reason, and the Doctrine of Method proposes a new method for moral education.
The Analytic, which is set up like a geometric proof, takes several steps to reach its primary conclusion, that the one ultimate moral principle is to only act such that the maxim of your will could hold universally. A law, Kant says, must be necessary and universal, for otherwise it is no law. If that is so, though, its force cannot be dependent on any contingent feature of the person following it. Next he argues that any law whose force was supposed to depend on its content would run afoul of this—if we tried to say that obedience to God was the ultimate moral law, we could not, for this law could only hold for those who wanted to obey God. Furthermore, on the view of human psychology Kant advances, to act on one's desire to be obedient to God would be to act to satisfy one's contingent pleasure in such obedience. This leaves only the empty form of universality to be the law-giving force of the law. So, for example, it is forbidden to break one's promises, since it would be impossible for breaking promises to be universalized.
The Analytic now goes on to argue that the free person and the moral person are one and the same. The free person acts on a law, and not randomly, but not an externally given law, for that would be a form of slavery. Only the categorical imperative is found suitable. Conversely, the moral person is following the practical law and is not bound by contingent desires, and so is autonomous.
The Dialectic accuses all previous ethical writers of having made the same mistake, the mistake of having regarded the morally worthy as aiming at the highest good instead of seeing the highest good as that which is aimed at by morality. These ethical systems were doomed to fail because the moral will cannot be constrained by an independent highest good, since for it to seek anything independent of itself would be to constrain its freedom. In this phenomenal world, furthermore, the highest good is not to be found. However, since following the practical law presupposes believing that its aim, the highest good, will be then achieved, reason requires us to believe the highest good is achievable. It turns out that this, in turn, requires belief in God and immortality. Without God, there is nothing to guarantee that following the moral law will produce the highest good of happiness proportional to morality, and without immortality, there is not enough time for us to achieve perfect morality.
We understand our freedom—which would otherwise be undetectable—while following the moral law. This following of the moral law frees us from the control of our desires. Our ability to feel the force of the practical law is also how we come to know that there is such a law. Therefore, the conclusions about this law, reached in the beginning of the Analytic, are not merely hypothetical. In arguing thus for the reality of morality and freedom, Kant reverses the order of evidence he had in his earlier Groundwork for the Metaphysics of Morals, where he derived morality from freedom.
Finally, in the Doctrine of Method, Kant proposes a method for teaching morality. It is essential to teach the student to act from duty, and not merely outwardly, conforming to morality. Kant recommends that we enlist our pupil's natural delight in arguing about ethical matters and allow him to develop his judgment by asserting various purported moral actions. We are warned not to either err by presenting examples of overblown heroism as paradigms of morality—since these will not help the student deal with normal, non- melodramatic moral dilemmas—or by presenting morality as prudent, since then the student will never learn to properly love morality for its own sake. By presenting examples of the moral law acting purely and without the help of other incentives, the student then learns to understand how the moral law can free him from slavery to his desires.