Immanuel Kant (April 22, 1724 - February 12, 1804) was a Prussian philosopher, generally regarded as the last major philosopher of the early modern period and 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 (see solar nebula).
Kant was born, lived and died in Königsberg. He spent much of his youth as a solid, albeit unspectacular, 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 this 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 he was in his late fifties before he did anything that would bring him 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 married and he owned only one piece of art in his household, advocating the absence of passion in favor of logic so that he may better serve. He never left Prussia, and rarely stepped outside his own home town. However, despite his reputation of being a solitary man, he was considered a very sociable person: he would regularly have guests over for dinner, insisting that sociable company was good for his constitution, as was laughter.
Around 1770, when he was 46 years old, Kant read the work of the Scottish philosopher David Hume. Hume was fiercely empirical, scorned all metaphysics, and systematically debunked great quantities of it. His most famous thesis is that nothing in our experience can justify our assuming that there are "causal powers" inherent in things—that, for example, when one billiard ball strikes another, how can we assume the second one "must" move. Of course, things have always happened this way, and through "custom and habit" we tend to assume they will continue to do so, even though we have no rational grounds for the assumption. 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, he 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 perceive and think 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, we can know 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 two major works: the Critique of Practical Reason and the Critique of Judgement. The Critique of Practical Reason dealt with morality (action) in the same way that the first Critique dealt with knowledge, and the Critique of Judgement dealt with the various uses of our mental powers that neither confer factual knowledge nor determine us to action, such as 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.
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 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 metaphysics and epistemology
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 termed his critical philosophy "transcendental idealism." While the exact interpretation of this phrase is contentious, one way to start to understand it is through Kant's comparison in the second preface to the "Critique of Pure Reason" of his critical philosophy to Copernicus' revolution in astronomy. Kant writes: "Hitherto it has been assumed that all our knowledge must conform to objects. But all attempts to extend our knowledge of objects by establishing something in regard to them a priori, by means of concepts, have, on this assumption, ended in failure. We must therefore make trial whether we may not have more success in the tasks of metaphysics, if we suppose that objects must conform to our knowledge" [Bxvi]. Just as Copernicus revolutionized astronomy by changing the point of view, Kant's critical philosophy asks what the a priori conditions for our knowledge of objects in the world might be. Transcendental idealism describes this method of seeking the conditions of the possibility of our knowledge of the world.
Kant's "transcendental idealism" should be distinguished from idealistic systems such as Berkeley's. While Kant claimed that phenomena depend upon the conditions of sensibility, space and time, this thesis is not equivalent to mind-dependence in the sense of Berkeley's idealism. For Berkeley, something is an object only if it can be perceived. For Kant, on the other hand, perception does not provide the criterion for the existence of objects. Rather, the conditions of sensibility - space and time - provide the "epistemic conditions", to borrow a phrase from Henry Allison, required for us to know objects in the phenomenal 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 (Kantianism)
Kant develops his moral philosophy in three works: Fundamental Principles of the Metaphysic of Morals (1785), Critique of Practical Reason (1788) and 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 (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):
The first formulation (the Formula of Universal Law) says: "act only in accordance with that maxim through which you can at the same time will that it become a universal law."
The second formulation (the Formula of Humanity) says: "Act that you use humanity, whether in your own person or in the person of any other, always at the same time as an end, never merely as a means."
The third formulation (the Formula of Autonomy) is a synthesis of the previous two. It says that we should so act that we may think of ourselves as legislating universal laws through our maxims. We may think of ourselves as such autonomous legislators only insofar as we follow our own laws.
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).
Kant's moral philosophy has come under some criticism as his lectures on anthropology have become further studied. A small minority of critics have argued that statements such as "All races will be exterminated except for that of the Whites" and that Africans are born for slavery (Reflexionen, 878) indicate that he does not consider non-whites to be persons in any meaningful ethical sense. This interpretation is by no means dominant, and the most accepted interpretation is that these lectures represent prejudices rather than serious philosophical thought.
Critique of Judgment
Critique of Practical Reason
Critique of Pure Reason
Groundwork of the Metaphysics of Morals
Metaphysical Foundations of Natural Science
The Metaphysics of Morals
Prolegomena to any Future Metaphysics
Religion Within Limits of Reason Alone
Observations on the Feeling of the Beautiful and Sublime
Idea For A Universal History With A Cosmopolitan Purpose
An Answer To The Question: 'What Is Enlightenment?'
Perpetual Peace: A Philosophical Sketch
The Contest of Faculties