 Political philosophy is not only a branch of Kant’s practical philosophy, it strongly depends upon Kant’s core practical philosophy for its basis. Kant’s practical philosophy and the categorical imperative that governs it were intended to form the ground not only for what is thought today to be ethics proper but also for everything that broadly speaking had to do with deliberative human behavior. He defined practical philosophy as that concerned with “rules of behavior in regard to free choice”, as opposed to theoretical philosophy that concerned “the rule of knowledge” (Kant 27: 243). Practical philosophy provided rules to govern human deliberative action. The Groundwork for the Metaphysics of Morals provided Kant’s main arguments that the categorical imperative is the supreme rule for human deliberative action. In its Preface, he notes that the Groundwork is to be a preparatory book for a future Metaphysics of Morals. Twelve years later he published that Metaphysics of Morals in two parts, the “Doctrine of Right” and the “Doctrine of Virtue”. Both are equally parts of Kant’s practical philosophy, and both thus have the categorical imperative as their highest principle.