Kant sets out to save metaphysics, but his prospects are bleak. He is dealing with two terminal schools - the "absolute despotism" of Leibnizian-Wolffian metaphysics and the "nomadic tribes" of skepticism.[1] Both of these tendencies had by his time collapsed into nihilism. Empiricism, taken to its extreme by David Hume, reduced men to animals and thought to conditioning. Consistent and thoroughgoing Rationalism on the other hand could only lead to Spinozist pantheism, as Jacobi famously argued. In his lectures on the history of philosophy, Hegel defended Spinoza from charges of atheism - on the contrary "with him there is too much God," in fact there is no such thing as the finite and "all this that we know as the world has been cast into the abyss of the one identity." What these have in common is the denial of human freedom, which Kant set out to save and would later describe as the "cornerstone" of his entire system.
Kant's favorite game, the secret of his whole 'gentle critique,' is the doubling of dualities. This is how he overcomes the deadlock opposition of Empiricism and Rationalism. Two illustrative examples are his discovery of the synthetic a-priori and his description of space and time as both empirically real and transcendentally ideal.
Synthetic A-Priori
Prior to Kant, the pairs analytic-a-priori and synthetic-a-posteriori were taken as coextensive, particularly by Hume, who argued on this basis that all logical connection was (because synthetic) derived from experience (a posteriori). Famously he gave the example of billiard balls, and argued that even when we see one hit another and the the latter moves, we cannot derive cause and effect with any necessity because this assertion is based on a mere repetition of contingent experience; this leads ultimately to a a profound and convincing skepticism, tending towards a nihilistic sort of anthropology in which everything that makes us human is beat into us by the caprice of experience. Kant and Herder read Hume in the 1760s, and while the student pushed onward in this brave new world and became the father of modern anthropology and social science, the master withdrew, and began formulating a defense. It took him almost 20 years.
So the problem of pure reason is this: how can any reason be possible which is not reducible to experience, or itself reduced to the bare form of non-contradiction? Kant's answer is the synthetic a priori, which he arrives at by subverting the traditional divide between analytic-a-priori and synthetic-a-posteriori. He takes mathematics hostage, demonstrating in the introduction to the first critique that geometrical proofs and arithmetic sums are not analytic as supposed (do you really have the sum of 172128 + 8492817 contained in the concept of these numbers?), that they are synthetic, and since math is obviously not derived from experience. Kant has won a beachhead, from which he promptly deploys his forces, seizing first space and time in the Transcendental Aesthetic and later the categories in the Transcendental Logic. With this, metaphysics was (for the time being) saved.
Space and Time
Rationalists took the transcendentally transcendentally real as the object of their metaphysics - they sought to pierce the veil of sense with reason and grasp God, time, the transcendental order of reality. Kant is not interested in this object, and sets it aside as the 'thing in itself.' Empiricists on the other hand waged war on Reason's dogma until they found themselves in the ashes of the empirically ideal, that is, the Humean shadow realm of experience structured by nothing but repetition within its field. Kant takes this seriously, but limits it to the fleeting realm of sense-perception, of color, taste, card-tricks and rainbows. Space & Time are in the middle, empirically real but transcendentally ideal. By empirically real Kant means that within the realm of "all possible experience" space and time are real, that is to say that they are non-negotiable and constant throughout, making them valid ground for scientific judgement. But they are also transcendentally ideal; transcendental meaning that they transcend the bounds of our possible experience (do you ever experience pure or complete space or time?), are not reducible to or derived from that experience, but at the same time transcend only as limits transcend a set, they are not 'out-side' - they are ideal, that is, they are of relevance only to the operations of our mind, transcendent only as conditions of appearance, the realm to which we must limit ourselves.
- ↑ "At first, her government, under the administration of the dogmatists, was an absolute despotism. But, as the legislative continued to show traces of the ancient barbaric rule, her empire gradually broke up, and intestine wars introduced the reign of anarchy; while the sceptics, like nomadic tribes, who hate a permanent habitation and settled mode of living, attacked from time to time those who had organized themselves into civil communities. But their number was, very happily, small; and thus they could not entirely put a stop to the exertions of those who persisted in raising new edifices, although on no settled or uniform plan. In recent times the hope dawned upon us of seeing those disputes settled, and the legitimacy of her claims established by a kind of physiology of the human understanding—that of the celebrated Locke. But it was found that—although it was affirmed that this so-called queen could not refer her descent to any higher source than that of common experience, a circumstance which necessarily brought suspicion on her claims—as this genealogy was incorrect, she persisted in the advancement of her claims to sovereignty. Thus metaphysics necessarily fell back into the antiquated and rotten constitution of dogmatism, and again became obnoxious to the contempt from which efforts had been made to save it. At present, as all methods, according to the general persuasion, have been tried in vain, there reigns nought but weariness and complete indifferentism—the mother of chaos and night in the scientific world, but at the same time the source of, or at least the prelude to, the re-creation and reinstallation of a science, when it has fallen into confusion, obscurity, and disuse from ill directed effort." (CPR, preface)