The argument that Kant has set forth so far goes like this: (1) there is something like a causality by freedom which is distinct from natural causality; (2) that freedom is autonomy (defined as “the will’s property of being a law to itself”). The argument proceeds by adding two statements that are meant to represent the conclusion (by elimination again):
(P3) But the proposition, the will is in all its actions a law to itself, indicates only the principle, to act on no other maxim than that which can also have as object itself as a universal law.
As stated in the preceding section, a will that “is in all its actions a law to itself”, hence autonomous, is committed to act on a maxim which “can also have as object itself as a universal law” (G 447).
(P4) This, however, is precisely the formula of the categorical imperative and is the principle of morality; hence a free will and a will under moral laws are one and the same (G 447)
Kant's argument equates causality by freedom with causality by moral laws thus disclosing the core meaning of the Reciprocity Thesis. Is that equivalence sound?
An alleged problem noticed by Allison (in this book, pp. 287-288) with the derivation of (P2) from (P1) is that it requires only a weak concept of free will causation (see the previous post on "Groundwork part 2" for reference to P1 and P2). In these passages Kant endeavors to derive moral autonomy from heteronomy (the latter assimilated to causal determinism), albeit it can only be derived that all that coherently follows from heteronomy is clearly autonomy with the less substantive meaning of free agency in general. Is Kant incoherent here? Did he inadvertently stumble upon a fatal non sequitur? As Allison has observed, the problem is complex and involves reconsidering the relation of transcendental freedom with rational justification.
According to Allison, Kant is arguing that freedom and morality reciprocally imply one another. A different way to state the same point it might be to consider freedom as both necessary and sufficient condition of morality. The argumentative step Kant elaborates in these paragraphs is far from being a mere resume of the remarks formerly stated in the previous sections of the Groundwork. In order to prove that the supreme principle of morality (the moral law) is objectively valid for human beings (even if it must present itself in imperatival form due to their sensuous nature), Kant needs to demonstrate that human beings are capable of a particular kind of freedom, namely, positive freedom or spontaneity, which amounts in turn to prove that positive freedom is real. This way of considering Kant’s task clearly connects it with the Two Worlds argument, which is at the heart of Groundwork III. Kant himself seems to gesture at it in the third paragraph of the first section of Groundwork III, when he claims that in so far as the principle of morality is a synthetic proposition we need a third element to connect freedom, negatively conceived, and morality. The third element must be the positive concept of freedom, “which cannot be , as in the case of physical causes, the nature of the sensible world (in the concept of which the concepts of something as cause in relation to something else as effect come together)” (G 447). The issue is at the core of what Kant will discuss in the subsequent sections of Groundwork III. Here, however, he seems to deal with a slightly more ethereal problem: how freedom can be conceived as a sufficient condition of rational moral conduct. In order to understand the intelligibility of the Reciprocity Thesis we need to clarify this point.
It is plain enough that freedom, defined as the capacity of a rational agent not to be influenced by alien causes such as external pushes or internal impulses, is a necessary condition of morality, and Kant has took it for granted all along the first two sections. It is intuitively plain that moral actions wouldn’t come off animal or natural causes. This way of putting the issue only qualifies one side of the concept of freedom, the negative property of the will, such that “it can be efficient independently of alien causes”, while the other side, the more substantive one, remains mysterious.
My reconstruction will be necessarily tentative and sketchy. From what I can gather, Kant’s argument can be fully appreciated in the light of a triangle of intertwined theses: the Incorporation Thesis, the Reciprocity Thesis and the Justification Thesis. The Incorporation thesis states that rational agents possess transcendental freedom or spontaneity, which they employ in the process of deliberation by incorporating reasons for action in their maxims. The Reciprocity Thesis adds that this kind of freedom is identical to morality. The Justification Thesis claims that all rational agents, in order to be moral, have to choose maxims that conform to moral law because they conform. From these building materials it can be derived an argument which Kant didn’t explicitly make in that form, but might well represent his most considered view:
1. in order to act rationally an agent must possess transcendental freedom (he must act on the basis of reasoned choices instead of impulses);
2. transcendental freedom is therefore a necessary condition of moral conduct (without it both responsibility and imputability would cease to be intelligible to us);
3. because the principle that guides agent’s deliberation is the same principle that governs moral conduct, i.e., the categorical imperative, then…
4. freedom, in a transcendental sense, is also a sufficient condition of morality.
Admittedly, the argument is hardly unproblematic. At least point number 3, not to mention the rest, seems to need further refinement. By spelling out the intrinsic and unavoidable nexus between positive or transcendental freedom and morality, it opens up to the possibility of a constructivist interpretation of Kant’s ethics – see Korsgaard’s construal of the argument (of point 3 in particular) in The Sources of Normativity (pp. 92-100) and in Creating the Kingdom of Ends (pp. 159-183) – and to the related long-standing and ongoing dispute between realists and anti-realists. I’m not going to take part in the dispute, for now it suffices to acknowledge that in order to prove that moral law is valid, Kant needs only to prove that freedom can be a real possibility for human beings. In order to do that, as Kant will make clear in the subsequent section, freedom must be presupposed “as a property of the will of all rational beings”.