Kant was the decisive break with the Enlightenment [in its concept of and confidence in reason] and the first major step toward postmodernism. Contrary to the Enlightenment account of reason, Kant held that the mind is not a response mechanism but a constitutive mechanism. He held that the mind-- and not reality-- sets the terms of knowledge. And he held that reality conforms to reason, not vice versa. In the history of philosophy, Kant marks a fundamental shift from objectivity as the standard to subjectivity as the standard.
Wait a minute, a defender of Kant may reply. Kant was hardly opposed to reason. After all, he favored rational consistency and he believed in universal principles. So what is anti-reason about that? The answer is that more fundamental to reason than consistency and universality is a connection to reality. Any thinker who concludes that in principle reason cannot know reality is not fundamentally an advocate of reason. That Kant was in favor of consistency and universality is of derivative and ultimately inconsequential significance. Consistency with no connection to reality is a game based on subjective rules. If the rules of the game have nothing to do with reality, then why should everyone play by the same rules? These were precisely the implications the postmodernists were to draw eventually.
Kant was thus different from previous skeptics and religious apologists.* ... Kant's point was deeper, arguing that in principle any conclusion reached by any of our faculties must necessarily not be about reality. Any form of cognition, because it must operate a certain way, cannot put us in contact with reality. ...
Kant is a landmark in a second respect. Earlier skeptics had, despite their conclusions, continued to conceive of truth as correspondence to reality. Kant went a step further and redefined truth on subjective grounds. ...
With Kant, then, external reality thus drops almost totally out of the picture, and we are trapped inescapably in subjectivity-- and this is why Kant is a landmark. ONce reason is in principle severed from reality, one then enters a different philosophical universe altogether.
This interpretive point about Kant is crucial and controversial. An analogy may help drive the point home. Suppose a thinker argued the following: "I am an advocate of freedom for women. Options and the power to choose among them are crucual to our human dignity. And I am wholeheartedly an advocate of women's human dignity. But we must understand that the scope of a woman's choice is confined to the kitchen. Beyond the kitchen's door she must not attempt to exercise choice-- whether to cook or clean, whether to cook rice or potatoes, whether to decorate in blue or yellow. She is sovereign and autonomous. And the mark of a good woman is a well-organized and tidy kitchen." No one would mistake such a thinker for an advocate of women's freedom. Anyone would point out that there is a whole world beyond the kitchen and that freedom is essentially about exercising choice about defining and creating one's place in the world as a whole. The key point about Kant, to draw the analogy crudely, is that he prohibits knowledge of anything outside our skulls. He gives reason lots to do within the skull, and he does advocate a well-organized and tidy mind, but this hardly makes him a champion of reason. The point for any advocate of reason is that there is a whole world outside our skulls, and reason is essentially about knowing it.
Kant's contemporary Moses Mendelssohn was thus precient in identifying Kant as "the all-destroyer." Kant did not take all of the steps down to postmodernism, but he did take the decisive one. Of the five major features of Enlightenment reason-- objectivit, competence, autonomy, universality, and being an individual faculty-- Kant rejects objectivity. Once reason is so severed from reality, the rest is details-- details that are worked out over the next two centuries. By the time we get to the postmodernist account, reason is seen not only as subjective, but also as incompetent, highly contingent, relative, and collective. Between Kant and the postmodernists comes the successive abandonment of the rest of reason's features.
* It is popular these days among philosphy students haughtily to deny to those who haven't read the second preface and therefore are missing something about the context, that Kant means what he says in writing "I have therefore found it necessary to deny knowledge, in order to make room for faith. But the context is quite clear: this follows a section where he "saves" free will from the contradiction it supposedly encounters in reason (determinism on the model of physics) by saying that who knows what the soul is really like, once we admit we can't know things in themselves-- we can't conclude anything about actual things based on arriving at a contradiction. Then he says that the same reasoning can be applied to God and to our immortality, and grouping these with free will, makes it seem like a good idea to "[render] all practical extension of pure reason impossible." The infamous sentence directly follows. In the paragraph, he emphasizes that this is the most important part of his theory: "But, above all, there is the inestimable benefit, that all objections to morality and religion will be for ever silenced ... by the clearest proof of the ignorance of the objectors." The religious apologetics seem pretty clear, in context, to me.