How is the practical deduction possible?

Abstract

In the Critique of Pure Reason, Immanuel Kant asserts the impossibility of the principles of practical reason, as “the objective reality of the moral law cannot be proved by any deduction” (KpV, АА 05: 47). This is the case if deduction is understood as the procedure Kant follows in the “Transcendental Analytic” of the first Critique. Yet, Kant himself points out that “deduction […] is the justification (Rechtfertigung) of its objective and universal validity” (KpV, AA 5:46). This justification of the principles of practical reason can be found in Kant’s works and has a certain structure. Firstly, in the Critique of Pure Reason, he justifies the existence of an intelligible world where the determining ground of causality is freedom. In the intelligible domain, the principles of pure reason possess objective reality. Thus, in the first Critique, Kant justifies the existence of (i) an intelligible world, (ii) freedom as the ground of causality and (iii) the ought as a reason for practical action. Secondly, in the Groundwork of the Metaphysics of Morals, Kant demonstrates that freedom carries one to an intelligible world where the good will resides. The good will rests upon freedom and brings forth human autonomy. And it is the good will from which the categorical imperative and morality in general emanate. Thirdly, it is explicated in the second Critique that freedom is the keystone (Schlußstein) of practical reason. Here, the moral law (= “a fact of reason”) is instrumental in deducing transcendental freedom itself. This means that the reality of transcendental freedom manifests itself through the moral law. The moral law exists and is effective; therefore, there is transcendental freedom behind it. Thus, the structure of transcendental deduction of practical reason consists of the successive justification of (i) the intelligible world, (ii) freedom, (iii) the good will, (iv) duty (categorical imperative), (v) the moral law as a “fact of reason”. Consequently, the practical deduction is possible only through a synthetic union between the Copernican Revolution,transcendental idealism and criticism.