Immanuel Kant (1724-1804) was a German (Prussian) 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 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. His revolutionary pieces were written very late in life after a long period of silence.
Kant's philosophy in general
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 assumpton of natural forces that humans cannot sense, but are used to explain movement of physical bodies.
Kant's metaphysics and epistemology
Kant's most widely read and most influential book is Critique of Pure Reason (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 catagories 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's moral philosophy
Kant's moral philosophy is developed in three works: Grounding for the Metaphysics of Morals (1785) and Critique of Practical Reason (1788) and The Metaphysics of Morals (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. A hypothetical imperative, by contrast, is a conditional obligation, one that we have only if we have some desire or other: if one wants x, one ought to do y. Kant held that there was but one categorical imperative, which can be formulated in four (allegedly equivalent) ways, and all our obligations can be derived, somehow, from this. On the first and perhaps best-known formulation: "act only on that maxim by which you can at the same time will that it should become a universal law. According to another well-known formulation, we should: "Act in such a way that you always treat humanity, whether in your own person or in the person of any other, never simply as a means, but always at the same time as an end." (See categorical imperative.)
The notion that we have simple irreducible obligations, that cannot be explained in terms of the obtaining of pleasure, happiness, or any other consequences, 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).