Space is not the vague and undetermined medium which Kant imagined; if purely and absolutely homogeneous, it would be of no use, and could not be grasped by the mind. Spatial representation consists essentially in a primary coordination of the data of sensuous experience. But this coordination would be impossible if the parts of space were qualitatively equivalent, if they were really interchangeable.

Kant's philosophy shifts for the first time the whole of modern thought and being (Desein) into the clarity and transparency of the foundation (Begrundung). This determines every attitude toward knowledge since then, as well as the bounds (Abgrenzungen) and appraisals of the sciences in the nineteenth century up to the present time. Therein Kant towers so far above all who precede and follow that even those who reject him or go beyond him still remain entirely dependent upon him. p. 55-56

Kant's distinction between things in themselves and things as they appear to us has both epistemological and metaphysical components. He is committed to a genuine idealism about things as they appear to us, but this is not a phenomenalist idealism. He is committed to the claim that there is an aspect of reality that grounds mind-dependent spatio-temporal objects, and which we cannot cognize, but he does not assert the existence of distinct non-spatio-temporal objects.

Some forms of Kantianism put great weight on “what we can imagine” holding that this can be a source of insight into necessary connections. Thus various of Kant’s arguments about space and time depend on the purported fact that it is impossible for us to imagine certain things: we can know, Kant claims, a priori that space has only three dimensions because we cannot imagine it as having more than three dimensions. History in the form of non-Euclidean geometry and modern physics has put paid to that particular line of argument, but in general we should beware of depending too much on “What we can imagine?,” especially in politics. As Nietzsche puts it somewhere, sometimes the fact that you can’t imagine a situation in which things are very different from the way they are now is not an especially good argument for the claim that they must be as they now are, but, rather, represents a failure of your powers about which you should feel mildly apologetic.

Kant thought things, not because they were true, but because he was Kant.

The pure normative standpoint that Kant’s ethics tries to occupy, a standpoint in which we consider only the normatively relevant features of a possible world, abstracting strictly from the real world and the empirical accidents of concrete situations, is an expression of what Dewey called “the quest for certainty.” In an insecure world, weak humans struggle convulsively to reach some kind of stability; the a priori is an overcompensation in thought for experienced human weakness. This is one of the origins of Kant’s notorious rigidity, his authoritarian devotion to “principles,” and his tendency to promote local habits of thought to constituents of the absolute framework in which alone (purportedly) any coherent experience was possible; thus, Euclidean geometry is declared the a priori condition of human experience, and sadistic remnants of Puritanism become demands of pure practical reason. Classical liberalism rejected Kant’s practical philosophy, but perhaps this is not enough. Perhaps one should also reject the very idea of a pure normative standpoint.

It may seem as if Kant was content with such a radically dualistic view of human action, but ultimately he was not. … What Kant is assuming here is that morality is not just a matter of making rightful or virtuous choices, but also requires us to put those choices into practice by attempting to realize the goals or ends that they entail in the arena of action, that is, nothing less than the realm of spatial, temporal, and causal nature in which we live and act.

Indeed, once Kant had discovered the Humean problem of skepticism about the universality and necessity of first principles, he generalized it not only to the first principles of “speculative philosophy,” that is, theoretical cognition, but also to the first principle of practical philosophy, the fundamental principle of morality. Thus, from a methodological point of view, Kant’s project in philosophy became that of undermining both Humean and Pyrrhonian skepticism in both theoretical and practical philosophy, and, much more incidentally, along the way refuting Cartesian skepticism about external objects as a nagging but by no means central problem in theoretical philosophy.

As Voltaire once remarked, "It is the privilege of the real genius, especially one who opens up a new path, to make great mistakes with impunity." The Copernican revolution brought about by Kant was, I think, the most important single turning point in the history of philosophy. For that reason there has been, ever since, a watershed in understanding between those who have taken his work on board and those who have not. For a good many of the problems he uncovered, the solutions he put forward have not stood the test of time, but his uncovering of the problems remains the most illuminating thing a philosopher has ever done. Because of the fundamental character of these problems, and because Kant did not solve them, confronting them has been the most important challenge to philosophy ever since.

In this precise sense, Kant was "the inventor of the philosophical history of philosophy": there are necessary stages in the development of philosophy, that is, one cannot directly get at the truth, one cannot begin wit it, philosophy necessarily began with metaphysical illusions. The path from illusion to its critical denunciation is the very core of philosophy, which means that successful ("true") philosophy is no longer defined by its truthful explanation of the totality of being, but by successfully accounting for the illusions, that is, by explaining not only why illusions are illusions, but also why they are structurally necessary, unavoidable, and not just accidents.

Once the relation of science and metaphysics with “intellectual intuition” is misunderstood, Kant has no difficulty in showing that our science is entirely relative and our metaphysics wholly artificial. Because he strained the independence of the understanding in both cases, because he relieved metaphysics and science of the “intellectual intuition” which gave them their inner weight, science with its relations presents to him only an outer wrapping of form, and metaphysics with its things, an outer wrapping of matter. Is it surprising, then, that the first shows him only frameworks within frameworks, and the second phantoms pursuing phantoms? He struck our science and metaphysics such rude blows that they have not yet entirely recovered from their shock. Our mind would willingly resign itself to see in science a wholly relative knowledge and in metaphysics an empty speculation. It seems to us even today that Kantian criticism applies to all metaphysics and to all science. In reality it applies especially to the philosophy of the ancients, as well as to the form — still ancient — that the moderns have given most often to their thought. It is valid against a metaphysics which claims to give us a unique and ready-made system of things, against a science which would be a unique system of relations, finally against a science and a metaphysics which present themselves with the architectural simplicity of the Platonic theory of Ideas, or of a Greek temple. If metaphysics claims to be made up of concepts we possessed prior to it, if it consists in an ingenious arrangement of pre-existing ideas which we utilize like the materials of construction for a building, in short, if it is something other than the constant dilation of our mind, the constantly renewed effort to go beyond our actual ideas and perhaps our simple logic as well, it is too evident that it becomes artificial like all works of pure understanding. And if science is wholly the work of analysis or of conceptua

He [Kant] suggested that experience may be not at all simple, but always complex, so that the very possibility of the experience which seems to the empiricist the absolute foundation of knowledge may depend on the presence in it of a factor that will have to be acknowledged as a priori. This factor issues from the nature of the mind that has the experience, and introduces into experience all that distinguishableness, that arrangedness, and that describable form, without which it could not be conceived as apprehensible or intelligible, that is, as an experience at all.

It was in this attitude of faith as pure fealty to the moral ideal, that Kant left the human spirit at the close of his great labours. It was the only solution left him, after his thesis of the absolute limitation of knowledge to objects of sense. But surely that thesis has a strange sound, coming from the same lips that utter with equal emphasis the lesson of our really having cognitions that are independent of all experience. This is neither the place nor the time to expose the oversight and confusion by which Kant fell into this self-contradiction; I must content myself with saying that the contradiction exists, and that I think the oversight is exactly designable, and entirely avoidable. There is a truth concealed in Kant's thesis of the immutable conjunction of thought and sense, but there is a greater falsehood conveyed by it.

With these dramatic words, Kant alludes to the two great problems and accomplishments of his philosophical career. On the one hand, he wants to know how we who as creatures are a mere part of nature can discover how all of nature … does and even must work. On the other hand, he wants to display the unconditional value that we have as rational rather than merely natural beings, … and that we are always free to act in accordance with and indeed for the sake of this principle, thus free to realize the unconditional value for which we unlike anything else in nature have the potential.