Immanuel Kant (April 22, 1724 - February 12, 1804) was a German philosopher, generally regarded as the last major philosopher of the early modern[?] period and, on anyone's account, one of history's most influential thinkers.
Kant is most famous for his view—called transcendental idealism[?]—that we bring innate[?] forms and concepts to the raw experience of the world, which otherwise would be completely unknowable. Kant's philosophy of nature and human nature is one of the most important historical sources of the modern conceptual relativism[?] that dominated the intellectual life of the 20th century—though it is likely that Kant would reject relativism in most of its more radical modern forms. Kant is also well-known and very influential for his moral philosophy. Kant also proposed the first modern theory of solar system formation, known as the Kant-Laplace hypothesis .
Kant spent most of his life in Königsberg (now Kaliningrad). He spent much of his youth as a solid but not spectacular student, living more off playing pool than his writings. He was of the rather curious conviction that a person did not have a firm direction in life until their thirty-ninth year; when his came and passed and he was just a minor metaphysician in a Prussian University a brief mid-life crisis ensued; perhaps it can be credited with some of his later direction.
Kant was a respected and competent university professor for most of his life, although nothing he did until his late fifties would have gained him much if any historical repute. He lived a very regulated life: the walk he took at three-thirty every afternoon was so punctual that local housewives would set their clocks by him. He never left Prussia, and rarely stepped outside his own home town. Despite the reputation he has earned though, he was considered a very sociable person: he would regularly have guests over for dinner, insisting that sociable company was good for his constitution.
Around 1770, when he was forty-six, Kant read the work of the Scottish philosopher David Hume. Hume's was fiercely empirical, and scorned all metaphysics (as well as systematically debunking great quantities of it). His most famous thesis is that nothing in our experience can justify our assuming that there are "causal powers" inhering in things—that, for example, when one billiard ball strikes another the second one in any sense "must" move. Of course, things always have happened this way, and we tend through "custom and habit" to assume they will; but we have no rational grounds for doing so. Kant was profoundly bothered. He simultaneously found Hume's argument irrefutable and his conclusions unacceptable. For ten years he published nothing, and then in 1781 released the massive "Critique of Pure Reason," arguably the most significant single book in modern philosophy. In this he developed his notion of a transcendental argument to show that, in short, although we cannot know necessary truths about the world "as it is in itself," we are nonetheless constrained to perceiving and thinking about the world in certain ways: we can know with certainty a great number of things about "the world as it appears to us": for example, that every event will be causally connected with others, that appearances in space and time will obey the laws of geometry and arithmetic, and so forth.
Over the next twenty-odd years until his death in 1804 Kant's output was unceasing. His edifice of Critical Philosophy was completed with the Critique of Practical Reason, which dealt with morality (action) in the same way that the first Critique dealt with knowledge; and the Critique of Judgment, which dealt with the various uses of our mental powers that neither confer factual knowledge nor determine us to action: aesthetic judgment (of the beautiful and sublime) and teleological judgment (construing things as having "purposes"). As Kant understood them, aesthetic and teleological judgment connected our moral and empirical judgments to one another, unifying his system.
Two shorter works, the "Prolegomena to any Future Metaphysics" and the "Groundwork to the Metaphysics of Morals" treated the same matter as the first and second critiques respectively, in a more cursory form—assuming the answer and working backward, so to speak. They serve as excellent introductions to the critical system. The epistemological material of the first Critique was put into application in the "Metaphysical Foundations of Natural Science"; the ethical dictums of the second were put into practice in "Metaphysics of Morals."
Aside from this Kant wrote a number of semi-popular essays on history, politics, and the application of philosophy to life. When he died he was working on a projected "fourth critique," having come to the conviction that his system was incomplete; this incomplete manuscript has been published as "Opus Postumum". Kant died in 1804.
Though he adopted the idea of a critical philosophy, the primary purpose of which was to "critique" or come to grips with the limitations of our mental capacities, Kant was one of the greatest of system builders, pursuing the idea of the critique through studies of metaphysics, ethics, and aesthetics.
One famous citation, "the starry heavens above me and the moral law within me", sums up his efforts: he wanted to explain in one systematic theory, those two areas or realms. Isaac Newton had developed a theory of physics that Kant wanted to build his philosophy upon. This theory involved the assumption of natural forces that humans cannot sense, but are used to explain movement of physical bodies.
His interest in science also led him to propose in 1755 that the solar system was created out of a gas cloud in which objects condensed due to gravity. This hypothesis is widely regarded as the first modern theory of solar system formation and is the ancestor to current theories of stellar formation.
Kant's most widely read and most influential book is Critique of Pure Reason[?]  (http://eserver.org/philosophy/kant/critique-of-pure-reason.txt) (1781), which proceeds from a remarkably simple thought experiment. He said, try to imagine something that exists in no time and has no extent in space. The human mind cannot produce such an idea—time and space are fundamental forms of perception that exist as innate structures of the mind. Nothing can be perceived except through these forms, and the limits of physics are the limits of the fundamental structure of the mind. On Kant's view, therefore, there are something like innate ideas[?]—a priori knowledge of some things (space and time)—since the mind must possess these categories in order to be able to understand the buzzing mass of raw, uninterpreted sensory experience which presents itself to our consciousness. Secondly, it removes the actual world (which Kant called the noumenal world[?], or noumena) from the arena of human perception—since everything we perceive is filtered through the forms of space and time we can never really "know" the real world.
Kant had wanted to discuss metaphysical systems but discovered "the scandal of philosophy"—you cannot decide what the proper terms for a metaphysical system are until you have defined the field, and you cannot define the field until you have defined the limit of the field of physics first. 'Physics' in this sense means, roughly, the discussion of the perceptible world.
Kant develops his moral philosophy in three works: Fundamental Principles of the Metaphysic of Morals  (http://eserver.org/philosophy/kant/metaphys-of-morals.txt) (1785), Critique of Practical Reason[?]  (http://eserver.org/philosophy/kant/critique-of-practical-reaso.txt) (1788) and Metaphysics of Morals[?]  (http://eserver.org/philosophy/kant/intro-to-metaphys-of-morals.txt) (1798). Under this heading Kant is probably best known for his theory about a single, general moral obligation that explains all other moral obligations we have: the Categorical Imperative.
A categorical imperative, generally speaking, is an unconditional obligation, or an obligation that we have regardless of our will or desires (contrast with hypothetical imperative).
Our moral duties can be derived from the categorical imperative. The categorical imperative can be formulated in three ways, which he believed to be roughly equivalent (although many commentators do not):
Example of the first formulation: If I breathe air, and I can will it so that everyone breathes air, we can see that breathing air is a moral obligation.
Example of the second formulation: If I steal a book from you, I am treating you as a means (to get a book) only. If I ask to have your book, I am respecting your humanity (or ability of rational thought).
The theory that we have universal duties, which hold despite one's subjective (and thus, merely hypothetical) imperatives that seek to fulfill one's own inclinations or happiness instead of these duties, is known as deontological ethics[?].. Kant is often cited as the most important source of this strand of ethical theory (in particular, of the theory of conduct, also known as the theory of obligation).
The amount of literature on Kant is ever-growing. Often, the best places to start are the introductions of his translated works. Modern translations usually suggest a variety of secondary literature, the purpose of which is both to explain and to interpret Kant's philosophy. For an example, see Christine Korsgaard's introduction to Mary Gregor's translation of the Groundwork, which not only provides a concise overview of Kant's moral philosophy, but also places his ethics within the framework of the larger critical system.
One of the best pieces of secondary literature on Kant's moral philosophy is a work by Korsgaard called "Creating the Kingdom of Ends". In this collection of essays, Korsgaard attempts to organize Kant's ethics into a coherent interpretation that may respond adequately to the modern defenders of ethical systems contrary with Kant's, such as Aristotle's, Hume's, and Hegel's.
Another good starting point of investigation is John Rawls' book of published lecture notes, titled "Lectures on the History of Moral Philosophy". The work is particularly useful in its investigation of Kant's moral philosophy within the vicissitudes of ethical systems from Hume to Leibniz to Hegel. Two other important scholars of Kant are Henry Allison[?] and Onora O'Neill[?]. Both authors have written books about Kant's moral philosophy.
German texts on the Internet (Kant himself)