THE CRITIQUE is primarily a book about metaphysics (as can already be seen from the thematic focus of its two prefaces, for instance). Accordingly, the types of skepticism that did play a crucial role in the origination of the critical philosophy and that it is mainly concerned to address are instead certain types of skepticism which threaten metaphysics specifically.
What was “metaphysics” for Kant? The discipline’s origins of course lay in Aristotle’s Metaphysics, which had contained two different conceptions of it in uneasy combination with each other: a conception of it as a science of being as such (book gamma), and a conception of it as a science of those beings which have only form, not matter (book lambda).1 After a long and complicated intervening history, the discipline had come to play a large role in German academia during the seventeenth and eighteenth centuries, taking a variety of forms.2 For most of the Germans in question, and in particular for the Leibniz-inspired authors of the eighteenth century who immediately preceded Kant, including Wolff and Baumgarten, as well as for the precritical Kant himself until about 1765, the discipline comprised two parts, roughly corresponding to Aristotle’s two original conceptions. First, there was “general metaphysics,” or “general ontology.” This gave an account of our most general concepts and principles, those that applied to all beings. Prominent examples were the concepts of substance and accident, and cause and effect, and the principle of sufficient reason. Second, there was “special metaphysics.” This dealt with three supersensible beings: the world as a whole, the human soul, and God.3
In the Critique Kant famously gives an account of progress in relation to the discipline of metaphysics according to which the inquiry begins dogmatically, then turns skeptical, before finally becoming critical.4 This account should be understood not only as history but also as autobiography.
Now we can approach the question of the specific types of skepticism concerning metaphysics which were crucial for the origination and motivation of Kant’s critical philosophy via an exegetical puzzle. In two different places Kant gives two different and seemingly incompatible accounts of what originally woke him from the “slumber” of “dogmatic” metaphysics and set him on the path towards the critical philosophy. In the Prolegomena of 1783 he famously writes: “I openly confess: David Hume’s reminder was the very thing which many years ago first interrupted my dogmatic slumber and gave my investigations in the field of speculative philosophy a quite new direction.”5 On the other hand, in a letter to Christian Garve from 1798 he states that it was the Antinomies that played this role: “The antinomy of pure reason—‘The world has a beginning; it has no beginning, and so on,’ right up to the fourth [sic]: ‘There is freedom in man, versus there is no freedom, only the necessity of nature’—that is what first aroused me from my dogmatic slumber and drove me to the critique of reason itself, in order to resolve the scandal of ostensible contradiction of reason with itself.”6
Faced with the apparent inconsistency between these two passages, one commentator (Lewis White Beck) in effect resorts to the hypothesis that in the latter passage Kant is senile!7 However, this explanation is surely desperate and implausible. For one thing, Kant had already made essentially the same point as he makes in the latter passage in the earlier and more formal context of his Prize Essay on Progress in Metaphysics of 1791 (henceforth: Prize Essay).8
The proper explanation is more complicated and interesting: Both passages contain a large and important measure of truth. They refer to two different, but equally significant, historical steps which Kant took in his protracted escape from the clutches of dogmatic metaphysics towards the (supposed) safe haven of the critical philosophy. Each step consisted in his recognition of, and reaction to, a kind of skepticism which he saw confronting dogmatic metaphysics, though the kinds of skepticism involved in the two cases were very different. The letter to Garve refers to an encounter with Pyrrhonian equipollence skepticism which occurred in the mid-1760s, whereas the passage from the Prolegomena refers mainly to an encounter with Humean skepticism concerning the possibility of a priori concepts and synthetic a priori knowledge which occurred either in or shortly after 1772. Let me explain, beginning with the earlier encounter alluded to by the letter to Garve.