Prolegomena to Any Future Metaphysics, by Immanuel Kant
1783, 301 pp.
This slender, concise work, which followed Kant's Critique of Pure Reason by two years, serves admirably its purpose as an efficient introduction to Kant's philosophy. It is structured with rigorous, meticulous logic: first an introduction to the problems of metaphysics, a very brief summary of the key points of the Critique, then an exploration of a priori (or "synthetic") reasoning in its various and increasingly complex forms. His purpose is nothing less than to construct a method by which metaphysics can be considered and practiced as a rigorous science, and slightly to persuade the reader to go and buy his previous book. He was apparently provoked to write the Prolegomena by a poorly-reasoned review of the Critiquein the Göttinger Gelehrten Anzeigen: he spends a bit of time in the Appendix ridiculing the totally forgotten fellows who misunderstood him.
As ever, Kant's diction is wildly difficult to follow. You need a machete and a team of sherpas to hack your way through sentences like this one: "Let the concept be that of cause, then it determines the intuition which is subsumed under it, e.g., that of air, relative to judgments in general, viz., the concept of air serves with regard to its expansion in the relation of antecedent to consequent in a hypothetical judgment." But at least here he is writing to be understood by the non-specialist and though at times he lapses into obscurity, his effort to be clear shows and is greatly understood.
Luckily the introduction is relatively painless and even rather amusing, and Kant's train of thought is actually quite easy to follow. He is extremely adept at explaining a position, but with one obvious problem, then considering that problem and explaining the position it leads to, save for one obvious difficulty, and so forth. Once he has set up his distinction between analytical reasoning (in which the proposition is contained in the concept itself, like "all bachelors are unmarried") and synthetic reasoning (in which the proposition is not contained in the concept, like "My whiskey is on the floor") his progression through increasing complexity is actually fairly logical. First he manages to prove (convincingly) that mathematical reasoning is synthetic, which proves that synthetic reasoning is possible, leaving only the question of how to apply it to other fields. So far so good. Then he just barely proves that scientific reasoning can be synthetic, but here he begins to stretch. The trick is in the connection of empirical experience to a priori reasoning: "Before a judgment of perception," he writes, "can become a judgment of experience, it is requisite that the perception should be subsumed under some such a concept of the understanding." In other words, the only way we can make sense of our perceptions is by categorizing and analyzing them using general concepts like quantity, orientation, location, and so forth. Those concepts of the understanding are synthetic and universally valid, since they are the tools we apply to any experience in order to understand it. Therefore the properties of a specific thing are subsumed under the concept, and you've moved from analytic to synthetic knowledge. Kant kindly provides a table of all possible concepts of experience, which in a fit of modesty he says "shows an inherent perfection, which raises it far above every other table which has hitherto though in vain been tried or may yet be tried." His ego is a consistent source of comedy: he begins the book by stating that nobody has ever done metaphysics before, that he has invented it wholesale in the single greatest work of philosophical genius in human history, but nobody cared because the Critique was long and boring. In the appendix he compares himself favorably to Euclid, and states that any future metaphysics will simply be minor digressions down a trail he has blazed. He seems like a fun guy to have at a party.
At any rate, the point is that experience is not just an aggregate of disconnected sensory perceptions, but in fact requires synthetic unity of understanding. But here he begins to go too far out on his synthetic limb: he argues outright that it follows from this proposition that we do not derive universal laws from experience, but our experience determines the universal laws. Certainly it must be granted that our experience determines our understanding of those laws, but it cannot possibly be asserted that "quantity" (for instance) would vanish in the absence of people to experience it.
Since these concepts are universal and therefore common to all experience, it follows that reason is capable of a complete understanding of all things, which is in Kant's argument the purpose of metaphysics. This complete understanding transcends the actual experience of any given individual thing, therefore Kant's metaphysics are transcendental.
The trouble with Kant is that he really seems to have advanced philosophy as far as any human possibly could go without the help of science. Within the closed system Kant constructs, God is indeed the logical conclusion: God is for Kant an issue of practical necessity. "We must therefore accept," he writes, "an immaterial being, a world of understanding, and a Supreme Being..because in them only, as things in themselves, reason finds [its] completion and satisfaction." Though the reader may despair at this cop-out after so much rigor, he at least seems to have a robust attitude towards attempting to understand the properties and relations of his Supreme Being. But he's arrived at this Supreme Being by following ontological predicates: unfortunately, I may have all the ontological predicates I want concerning the existence of a bottle of whiskey in the bottom drawer of my desk at work, but none of them will make it actually exist, and despite all of my clever reasoning, I will only be able to ascertain its (non)existence using verifiable science. Science was not in 1783 what it is today, though, so it is difficult to fault Kant too much. One wonders what he would have done with the extensions of science and the philosophy of science which have been developed in the interim.
The volume of the Prolegomena I read (the Carus translation from 1905) kindly included a discussion of Kant's life and work, a series of essays on Kant from a variety of philosophical standpoints, a sample of Kant's handwriting, and a translation of the original review which made him so angry. All of this was quite interesting, although the Prolegomena stand well on their own. The actual Prolegomena run only 160-odd pages, so these supplementary materials here make up about half the book and while they are probably quite useful to a new student of philosophy, to the initiated they mainly serve to assure you that other people find Kant difficult to understand as well. Essential for understanding Kant, and an excellent introduction to his philosophy, this work is highly recommended.