“..a free will and a will under moral laws are one and the same” (G 447).
This is one of the most striking and revolutionary claims in the entire history of moral thought. Its everlasting significance is widely recognized among contemporary moral philosophers who continue to argue about the proper way to understand it. In order to clarify the claim, which I believe it summarises the overall meaning of Kant’s ethics, I shall undertake a comment of the Groundwork of the Metaphysics of Morals, proceeding from the last section to the first – so reversing Kant’s original order of exposition – and commenting on it paragraph by paragraph from beginning to end [unless differently specified, I’ll refer to Mary Gregor’s translation and page numbers of the Academy Edition].
In Section III of the Groundwork, Kant lays out a sketch of what he calls a “critique of pure practical reason”. In the previous two sections, the concept of duty was elucidated with regard to that of good will, following a merely analytical method. In fact, the central argument of Section II was dedicated to the explanation of “the generally received concept of morality” (G 445) in order to find the supreme principle of it, namely autonomy, which has been then made intuitive by reference to the three formulations of the Categorical Imperative. Thus, while in the first two sections Kant uses the analytical method to explicate the relevant concept, now he is concerned with the “transition” from a mere analytic to a “synthetic use of pure practical reason” (G 445).
At the beginning of the first section, Kant makes it clear that the method he intends to follow proceeds “analytically from common cognition to the determination of its supreme principle, and in turn synthetically from the examination of this principle and its sources back to the common cognition in which we find it used.” (G392). Analytic and synthetic here neither refer to analytic and synthetic judgments, nor to the alleged content of the discovered principles, but to “regressive” and “progressive” ways of inquiry.
The analytic method, insofar as it is opposed to the synthetic, is something completely different from a collection of analytic propositions; it signiﬁes only that one proceeds from that which is sought as if it were given, and ascends to the conditions under which alone it is possible. In this method one often uses nothing but synthetic propositions, as mathematical analysis exempliﬁes, and it might better be called the regressive method to distinguish it from the synthetic or progressive method (Prolegomena, 276n).
According to what Kant specifies in the Prolegomena, a regressive argument starts from a concept (in the Groundwork, the good will) to disclose its condition or principle (the categorical imperative), therefore availing itself of conceptual analysis. By contrast, a synthetic or progressive argument goes from some premise to its consequences. This means that the line of reasoning Kant develops in the Groundwork can be represented as a circle: once we have elucidated the common concept of morality by disclosing its principle, then we can proceed from the discovered principle back to its application to common moral cognition. The four examples provided by Kant in the second section are meant to close the circle by offering a detailed discussion of how the various formulations of the principle apply to concrete moral circumstances. According to this interpretation, “analytic” refers to the first and second section, whereas “synthetic” applies to the central part of the second.
One drawback of this exegesis is that it leaves the role of the third section unexplained, which thus seems to remain out of the circle. Moreover, at the end of the second section Kant introduces a rather different characterization of the distinction between “analytic” and “synthetic”:
By explicating the generally received concept of morality we showed only that an autonomy of the will unavoidably depends upon it […] This section then, like the first, was merely analytic. That morality is no phantom - and this follows if the categorical imperative, and with it the autonomy of the will, is true and absolutely necessary as an a priori principle – requires a possible synthetic use of pure practical reason, which use, however, we cannot venture upon without prefacing it by a critique of this rational faculty itself… (G 445)
In this passage, Kant clearly states that the first two sections are to be understood as part of the “analytic” argument. Accordingly, we might restate the overall structure of the Groundwork as follows: having accomplished the first task, namely, “the search for the supreme principle” (G 392), Kant now turns to the second: proving its reality, which in turn means to demonstrate that the principle previously discovered is binding for all rational beings, in particular for humans. In other words, proving the objective validity of the principle requires a “transition” from the merely analytical standpoint of the metaphysics of morals to the synthetic use of reason entailed in the new standpoint of the critique. The present task is necessary in order to dispel the fears that the concept of morality explicated so far would turn out to be a mere “phantom of a human imagination” (G 407) or a “chimerical idea” (G 445).
As far as I can see, it follows that the overall movement of Groundwork’s argumentation should be understood as a spiral rather than as a circle, as when in a symphony one theme constantly returns but with different meaning. In the same way, along the course of the Groundwork, Kant repeatedly returns to common moral cognition in order to test what his a priori discourse has just acquired: (i) in the first section he takes the concept of good will to be completely in “agreement” with “common understanding” (G 394); (ii) in the second, he shows that the categorical imperative is the “canon” which lies at the basis of the common way of thinking about ethical issues (see the discussion of the four examples G 421-424, G 429-430); finally, (iii) in the third section, he comes up with the lapidary “The practical use of common human reason confirms the correctness of this deduction” (G 454), meaning that only the common moral judgment can definitely assure us about the correctness of the ultimate a priori justification of moral principle. If this intuition were correct, then it would confirm again, if it were needed, that Kant’s moral theory is deeply rooted in the Enlightenment’s concept of reason as sensus communis (you can find a brilliant discussion of this idea in Arendt’s lectures on Kant’s political theory).