Guyer, Paul, Kant's System of Nature and Freedom, Oxford: Oxford University Press, 2005.

Excerpt:

“Was Kant a systematic philosopher? This is not an easy question to answer. Although he certainly borrowed many aspects of the outward organization of his critical philosophy from his great predecessor Christian Wolff, who covered all of the topics of philosophy in detail 1st in 9 German volumes and then in 25 laps and volumes, and never claimed that any of his own 3 critiques constituted more than the preliminaries to a genuine system of philosophy in the Wolffian sense.  And he was not regarded as a systematic philosopher by such illustrious successors as Fichte, Reinhold, and Hegel,  all of whom held that a genuinely systematic philosophy must be derived from a single principle, the possibility of which Kant strenuously denied. Contrary to his successors Kant insisted that human thought is inexorably riven by fundamental dualities—the distinction between sensibility and understanding, the distinction between phenomena and noumena, or appearance and reality, and above all the distinction between theoretical and practical reason. It can often look as if Kant thinks that rhetorical reasoning and practical reason constitute 2 separate domains of human thought that cannot possibly be joined in a single system: in theoretical reasoning we use the pure forms of sensibility and understanding, that is our pure intuitions of the structure of space and time on the one hand and the fundamental logical structures of the discursive thoughts on the other, to define the basic laws of a realm that cannot be influenced by our moral conceptions of how things ought to be, while we  appeal to pure practical reason to determine how truly free beings ought to relate to themselves and one another regardless of what they actually do. Thus it can seem as if in Kant’s  view the realms of nature and freedom, while each possesses its own kind of systematic laws and organization, cannot be joined in a single system.”

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