Kant is considered one of the greats, because he was able to address complex questions with humility. Although he stakes serious claims, the claims are often built from humble premises. He takes ideas from all the schools of thought of his day and synthesizes them into one theory about reality. Broadly speaking, his theory can be understood as a kind of gentleman's agreement with the universe. He does what he can through empiricism and rationality, but when observation or logic fail to capture the truth of the situation, he simply observes that fact and leaves the mystery in tact.
Many have called Kant a transcendentalist or a mystic, but his theory can be viewed the opposite way just as easily. On could say he was pragmatic and practical. In "The Critique of Pure Reason," Kant famously explains that there is a limit to what can be treated through Empiricism and Rationalism. This has often led people to categorize him as a mystic (perhaps appropriately), but there is not doubting that the essay itself is pragmatic and practical. Because he understands the technical value of Empiricism, he is comfortable observing that not everything is observable to humans, and because he understands the technical value of rationality, he is comfortable saying that "Pure Reason" alone does not suffice to answer the questions of existence.
This leads to his famous discussion of ethics which can be found in "Groundwork of the Metaphysics of Morals." Instead of understanding the morality of mankind to be dependent on religion or religious belief, he observes that there are tangible aspects within morality that can guide that discussion better. In other words, he is writing about descriptive morality (often analyzing a given situation from various points of view), and ultimately, he suggests that perhaps the reality of human morality is more mystic than it is rational. He mentions categorical imperatives and their relationship to human instinct. To understand his point of view on this better, one should remember that Kant views good within a hierarchy.
By addressing Kant's essays "A Priori" and "A Posteriori," one can see the schism of Kant's philosophy in its technicality. Basically, these two categories are the general 'buckets' where he throws issues. Either an idea is "a priori," which means it is a statement that must be taken as true without evidence, or it is "a posteriori," which means that it follows a series of logical conclusions. This is Kant's primary approach to logic, and the relationship between the two categories explains much of his point of view. He worries that although many ideas are proven, if a person follows the conclusion far enough, they will see that most generally held beliefs are fixed on "a priori" beliefs that a person takes on faith. He argues in "Critique of Judgment" that even logic and Rationalism are fixed on an "a priori," that the universe is observable and discernible to the human mind.
