Kant sketches out here what is to follow. Most of these two chapters focuses on comparing the situation of theoretical and of practical reason and therefore discusses how the Critique of Practical Reason compares to the Critique of Pure Reason.
The Critique of Pure Reason was a critique of the pretensions of pure theoretical reason to attain metaphysical truths beyond the ken of applied theoretical reason. Its conclusion was that pure theoretical reason must be restrained, because it produces confused arguments when applied outside its sphere. However, the Critique of Practical Reason is not a critique of pure practical reason, but rather a defense of it as being capable of grounding behavior superior to that grounded by desire-based practical reasoning. It is a critique, then, of applied practical reason's pretensions. Pure practical reason must be restrained but rather cultivated.
Kant tells us that while the first Critique presented God, freedom, and immortality as unknowable, the second Critique will mitigate that claim. Freedom is knowable because it is revealed through the force of the moral law. God and immortality are not, but now (practical) reason requires belief in them. One might still be dissatisfied, wanting, say, proof of God's existence. Kant here invites his dissatisfied opponent to actually provide such a proof, believing that none is forthcoming. The discussion of freedom Kant holds to be especially important, for empiricists insist on thinking of it as a purely psychological thing in the phenomenal world, a complete confusion according to Kant.
The Critique of Practical Reason can stand alone from the earlier Groundwork for a Metaphysics of Morals, although it addresses some criticisms leveled against that work. In particular, Kant will address the issue of why he did not first discuss the highest good and then define the moral law in terms of it. A complete classification of duties will not occur in the second Critique because such a classification depends on how people contingently are. This work will proceed on a higher level of abstraction.
While valid criticisms against the Groundwork of the Metaphysics of Morals are to be addressed, Kant excoriates those criticisms he does not find helpful. He suggests that some of the gaps reviewers find in his arguments are in fact only in their brains, which are too lazy to grasp his ethical system as a whole. As for those who charge him with writing incomprehensible jargon, he challenges them to find more suitable language for his ideas, or otherwise to prove that they really are meaningless. Fortunately, Kant reassures us, that while the speculations of the first Critique required language very unlike common speech, this will be less true in the second Critique.
Finally, the sketch of the second Critique is presented in the Introduction. It is modeled on the first Critique. First, the Analytic will investigate the operations of the faculty in question. Next, the Dialectic will investigate how it can go astray. Finally, the Doctrine of Method follows, which will only be loosely analogous to its corresponding first Critique section, discussing how to bring about psychological influence of pure practical reason.
Kant's comparison of the first and the second Critique in the preface and his subsequent discussion in the introduction bring out one of the oddities of Kant's writing: a tendency to model his works after one another. Here it is questionable whether the structure of the first Critique was really so suitable for this book, and whether the parallels he discusses are more illuminating or more distracting. The first Critique utilizes theoretical reasoning—roughly, philosophical thinking—to examine the limits of the potential achievements of such thinking. The second Critique, however, as Kant points out, does not use pure practical reason—decision-making based on reason and not on desire—to point out the limitations of such decision- making. For one thing, it is unclear how one could "apply" a faculty of decision-making in a book, which is better seen as a recording of theoretical reason's activity.
Mainly though, Kant is not critiquing pure practical reason but lauding it, saying that it is possible and that it is the ground of morality. True, we can say that he is thereby attacking impure practical reason. Kant believes that although his beliefs about pure practical reason are commonsensical, insofar as common sense can grasp them, philosophers are liable to go astray and enshrine the self-serving calculations of impure practical reason in the place of pure practical reason. But it remains to be seen whether anything is really gained by setting up the analogy in the first place.
A point about the comparison which is important to remember is that the Critique of Practical Reason does not simply contrast with the Critique of Pure Reason, in that it critiques the impure reason which the first Critique still left unexamined. Rather, the title of the first Critique is meant to be understood as elliptical for "The Critique of Pure Theoretical Reason," while the title of the second Critique can be understood as elliptical for "The Critique of Impure Practical Reason". The pure / impure distinction, which has to do with whether contingent, sensory factors are involved, is not the same as the theoretical / practical distinction, which has to do with the faculty of knowing versus the faculty of acting.
This book contains three sections: the Analytic, the Dialectic, and the Doctrine of Method. The Analytic presents, in both critiques, the operations of the faculty in question. In the case of the second Critique, this will turn out to be a derivation of the one principle of pure practical reason, the categorical imperative, and an argument that obeying it is equivalent to freedom. The Dialectic presents, in both Critiques, arguments that the faculty in question can go astray. In the case of the second Critique, this will be an argument that pure practical reason goes wrong when it seeks perfection in this world, as well as an argument that what we should instead do is seek perfection in the next world with God's help, making the assumption that immortality and God exist. The Doctrine of Method in the first Critique plans out the future sciences of pure theoretical reason; the Doctrine of Method in the second Critique plans out the future of educating people in the use of pure practical reason.
We are also set up for Kant's 1797 "follow-up", the Metaphysics of Morals. The Critique of Practical Reason contains the one true ultimate moral principle, the categorical imperative. However, there is no full discussion of its application. That is because Kant intends everything in the Critique of Pure Reason to proceed a priori, without any reference to what, as a matter of contingent fact, human nature happens to be like. Without such a theory, we cannot say what, concretely, our duties are. The role of the Metaphysics of Morals is to give such a theory.