In the A and B Prefaces to the Critique of Pure Reason, Kant lights on different emphases in explaining the mission of the Critique. These different emphases demonstrate how Kant has changed his conception of the purpose of the Critique, in the years between the first edition of 1781 and the second of 1787.
In the 1781 A Preface, Kant focuses on the promise of possibility of metaphysics. As he writes, the critique sets out to determine the possibility of “cognitions after which reason might strive independently of all experience, and hence the decision about the possibility or impossibility of metaphysics in general” (Critique of Pure Reason, 101). Here metaphysics denotes knowledge of the traditional metaphysical objects, such as God, freedom and soul, none of which are manifest within experience. What is more, the question of the possibility of metaphysics is open, undetermined. Although we presume Kant composed the Preface after he had completed the Critique, he holds out for its possibility. Yet in the Critique, he will sharply restricts the speculative function of reason, and he indicates as much later in the Preface when he says of those objects transcending experience that he must “admit that this wholly surpasses my capacity” (102). So what then does he think that metaphysics is? We ask this question because it seems that he is saying that metaphysics is something other than the knowledge of these objects beyond experience. The answer seems to be suggested in the beginning of the A Preface, in which he describes the dialectical destiny of reason—its “peculiar fate.”
The 1787 B Preface, by contrast, seems oriented towards different matters, particularly, in the question of the positive and negative consequences of the Critique (Prolegomena to any Future Metaphysics, 148). The question is, is the Critique merely negative in its restriction of speculative reason? Kant thinks resolutely not. Conversely, this is a positive effect, as he famously explains: “I therefore had to cast out knowledge in order to make room for belief” (150). In this respect, something completely new enters the orbit of the problem of reason’s critique (relative to the description of the A Preface), namely, that the critique and limitation of the speculative or theoretical function of reason complements, nay, makes possible, the extension of practical philosophy. In brief, only if reason is limited in its knowledge of God, soul or freedom—in which matters it always otherwise remains in contradiction—can it hold out for the possibility of a moral belief in the existence of these unknowable objects. Belief is only possible if knowledge is limited.
Thus, between these Prefaces something dramatic has occurred. The conclusions of the Critique remain largely the same. But the meaning of those conclusions has been transformed. In the A Preface, Kant metaphysics concerns the contradictions reason inevitably finds itself in. In the B Preface, metaphysics or speculative reason is to be limited for the sake of practical philosophy. What is the connection between these internal rational contradictions and the aspirations of practical philosophy?
