The first section of Groundwork III consists of three paragraphs, the first of which sets out some basic statements that Kant sees as necessary for the following arguments. The second paragraph addresses the theme encapsulated in the title “The concept of freedom is the key to the explanation of the autonomy of the will”, whereas the third paves the way for the subsequent sections discussing the central issue of justification of the supreme principle of morality as a priori “synthetic proposition” (G 447). This section is supposed to contain one of the central tenets of Kantian ethics, namely the so called Reciprocity Thesis, according to which freedom and morality are reciprocal terms (see Allison 1990, pp. 201-213 and Allison 2011, pp. 283-293).
(G 446, §1)
To start with, Kant states some preliminary definitions. Will is defined as “a kind of causality of living beings insofar as they are rational”, freedom as “that property (Eigenschaft) of such causality that it can be efficient independently of alien causes determining it” and, finally, natural necessity is the “property of the causality of all non-rational beings to be determined to activity by the influence of alien causes” (G 446). Such statements are clearly meant to be the building blocks of the argument deployed in the following paragraph, and each of them contains central concepts of Kantian thought, both theoretical and practical. In Section II, Kant had already defined the will of a rational agent as “the capacity to act in accordance with the representation of laws, that is, in accordance with principles”, thus equating it with practical reason itself: “The will is nothing other than practical reason” (G 412). Remember that freedom too figured in the main argument of Section II as a necessary presupposition of it. However, now Kant adds a further qualification connecting both terms to the concept of causality. This move signals that Kant is setting himself to undertake a completely different task.
(G 446, §2)
The second paragraph can be broken down into two parts which constitutes one major argument devoted to the Reciprocity thesis.
After having noticed that the above definition of freedom is “negative” and so “unfruitful” to gaining a priori access (“to cognize through reason and a priori” is the meaning of the German word einzusehen) to the very essence of freedom, Kant claims that from there “flows from it a positive concept”. But what is this new “positive concept of freedom” which flows from the negative one? And how does it flow from the negative? The two parts of the paragraph are meant to respond to these questions in turn.
To start with the first part, Kant places two premises (P1 and P2) which represents a subsidiary reflection about the relationship between “causality” and “law”.
That freedom by no means is without law has already been emphasized by Kant in the Critique of Pure Reason. There, Kant separated freedom from causality according to nature, and defined the former as a sort of spontaneous activity, “the faculty of beginning a state from itself” (A 533/B 561); but here, freedom is conceptualized as a sort of causality by laws which is different, and in contrast with, the causality of nature. It might appear odd to equate spontaneity and causality by laws, but, as Kant will make clear soon, this would be the clue to the understanding of the argument’s overall meaning. In this connection, it is worth noting that Kant is rejecting the view, dear to contemporary metaphysicians, that equates freedom to the mere capacity to do otherwise.
So, which kind of causality by laws is here at stake? Is there a causality by laws which is alternative to, and in opposition with, natural causality? The second premise goes like this:
Kant has already stressed that freedom “must instead be a causality in accordance with immutable laws but of special kind”, but in this passage he unexpectedly adds to the picture a well-known couple of concepts, heteronomy and autonomy. The difference between causality by freedom and causality by natural necessity was already stated in the above definitions and in previous sections of the Groundwork as well. Kant here is claiming that instead of being a sort of heteronomy, like natural causality, freedom must be autonomy, i.e. a distinct kind of causality which rests on a special kind of law. Kant uses the distinction between heteronomy and autonomy, previously made with reference to the principles of morality, to connect firstly natural causality to heteronomy (non-rational beings are influenced by alien causes), and then to isolate, and point out, the connection between free causality and autonomy: “what, then, can freedom of the will be other than autonomy, that is, the will’s property of being a law to itself?” (G 447). Having excluded that the ‘causality of will’ is based on alien forces, Kant concludes, arguing by elimination, that it must be guided by inner forces, that is, by self-imposed laws.