Kant on the Justification of Moral Principles

Research output: Contribution to journalReview articlepeer-review


In Groundwork III, Kant attempts to give a deduction of the categorical imperative. There is widespread disagreement as to how Kant's argument is supposed to proceed. Many commentators believe that Kant's deduction fails because some of its argumentative moves are unjustified. In particular, Kant makes a mistaken inference from theoretical freedom to practical freedom, and his axiological 'superiority claim' regarding the noumenal world's priority over the sensible world is unjustified. According to the standard incompatibilist story, Kant came to see that his deduction was flawed by the time he wrote the Critique of Practical Reason, at which point he claimed that the truth of the moral law does not require a deduction since it is a "fact of pure reason". The moral law is no longer the conclusion of his argument; instead, it functions as the premise of an argument that establishes our freedom. Other commentators endorse a compatibilist reading, according to which the justifications of the moral law in Groundwork III and the second Critique are compatible because Kant never attempted to give the strong kind of deduction that he rightly rejects in the second Critique. On the view I develop here, the particular argumentative moves that the standard incompatibilist takes issue with are not flawed and incompatible with Kant's second Critique. I argue for a compatibilist reading of these moves. I think the compatibilist is right to claim that the deduction Kant considered impossible in both the Groundwork and the second Critique is what I call a strong deduction. I also agree with compatibilists that the deduction he actually delivers in Groundwork III is only a weak deduction that makes use of a merely problematic conception of transcendental freedom. However, I do think that Kant's argument in Groundwork III remains question begging in the final analysis. The facticity claim in the second Critique, by contrast, can provide a non-question-begging account of moral obligation. Here, I agree with the optimistic incompatibilist, who views the argument in the second Critique as an improvement on his argument in the Groundwork. However, in my novel account of Kant's argument, I endorse what I call 'radical incompatibilism' because it concerns the roots of Kant's approach to the justification of the moral law. What is novel about my account is the claim that the deduction in Groundwork III rests on the false assumption that practical cognition, like theoretical cognition, requires a critique of pure reason. In the second Critique, Kant revised his argument because he realized that, in contrast to synthetic a priori judgments of theoretical cognition, the possibility of synthetic a priori judgments of practical cognition can be derived from the actuality of a "deed". With respect to pure practical reason, the second Critique proceeds metaphysically i. e. dogmatically rather than critically. Hence Kant came to view a deduction of the categorical imperative as unnecessary and abandoned the project of a critique of pure practical reason. We should, for this reason, resist the generality of Kant's claim in the first Critique to the effect that, for all synthetic judgments a priori, "if not a proof then at least a deduction of the legitimacy of its assertion must unfailingly be supplied" (KrV, B 286).

Original languageEnglish (US)
Pages (from-to)55-88
Number of pages34
Issue number1
StatePublished - Mar 20 2017


  • Deduction of Freedom
  • Deduction of the Categorical Imperative
  • Fact of Reason
  • Groundwork III
  • Kantian Ethics

ASJC Scopus subject areas

  • Philosophy


Dive into the research topics of 'Kant on the Justification of Moral Principles'. Together they form a unique fingerprint.

Cite this