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Kant took the concepts he developed in his “Critique of Pure Reason�� and applied them deductively and in reverse order in the “Critique of Practical Reason��. Reason is used to develop the categorical imperative from the freedom of the will; however three things-in-themselves are needed to be postulated in order to fully develop his moral theory: liberty, immortality of the soul, and God. No sentiments, interests, or experiences should play any part in developing or following the categorical imperative (i.e. its development should be synthetic a priori). The only feeling we should experience towards the categorical imperative is respect. Good and bad considerations should not play any role in the development of the categorical imperative, but should only follow from it.
In his “Critique of Pure Reason�� Kant argued that we should stay away from elaborating on any thing-in-itself like God, causality, liberty, and so on since an antinomy will ensue �� that is we cannot think of something that is a thing-in-itself as if it is a phenomenon. Accordingly, these concepts should stay empty. In the “Critique of Practical Reason��, these three things-in-themselves are reintroduced and even given some positive contents. Moreover, the Practical Reason is considered more fundamental and important when compared with the Pure Reason.
Crucial to Kant's argument is that a rational being belongs to the experienced natural causality (i.e. is a phenomenon), but also is a free will (i.e. is a thing-in-itself). In this way, a rational being is placed in nature and thus under its deterministic laws, but also capable to initiate free will and thus under the moral law.
I enjoyed this book more than I expected; however Kant is quite boring, didactic, and conservative. Moreover, his excessively rational, systematic, axiomatic, deductive, lengthy, and repetitive approach seems to rather conceal the topic instead of illuminating it.