He lived his entire life in the provincial East Prussian university city of Koenigsberg (now Kaliningrad), first as a student, then as a professor, and in his greatest work, The Critique of Pure Reason, pulished in 1781, produced a theory of metaphysics that he believed to be a "Copernican revolution1" in philosophy.
In the mid eighteenth century European philosophy was dominated by two competing schools of thought, Empiricism and Rationalism.The empiricists, notably the English philosopher John Locke (i632-1704), and the Scottish philosopher David Hume (1711-1776), held that the mind is a tabula rasa, a blank slate on which knowledge of the external world is inscribed, and that our perceptions represent the world more or less as it actually is. Unfortunately, as Locke was forced to conclude, there can be no confirmation, independent of those same perceptions, that this is true.The rationalists, like Rene Descartes (1596-1650) and Gottfried Leibniz (1646-1716), believed on the other hand that Absolute truths, particularly of mathematics, geometry, and logic could be arrived at by thought alone, but in that case how could their applicability to the external world be guarenteed?
Kant's resolution of the paradox was a metaphysical system he called Transcendental Idealism. By this rather formidable term he meant that rather than our perceptions/cognitions being conformed to their objects, which could never be demonstrated, those objects necessarily conformed to the forms of our perceptions/cognitions, which were conceptions, or principles of organisation, supplied by the mind itself. Only the content that filled up these forms was given by the objects themselves.
For perceptions these forms were space (the form of external sense) and time (the form of internal sense). Kant showed that space and time could not be derived from our experience of the relations of objects in the world, for those relations were always given in terms of space and time, and could not be comprehended without them. Space and time, on the other hand, could be imagined without any objects present in them.
For cognitions the forms were primarily logical, together with conceptions of number, substance, and cause and effect. It was because these comprised the organisation of the understanding that the laws of arithmetic, geometry and physics had the necessity that mere empirical observation could not give them.
Noumenon and Phenomenon
For many people the most counterintuitive aspect of Kant's metaphysics is the claim that since the organisation of our perceptions/cognitions is contributed by the subject, in the absence of the subject none of that structure will remain. We cannot, therefore, have any knowledge of the world as it is in itself, apart from our representations of it. This world of things as they are in themselves Kant called the Noumenon, distinguishing it from the Phenomenon, the world of appearances, of which alone we could have knowledge.
Transcendental Idealism does not, however, thereby deny the real existence of the world, as some of Kant's successors seemed to have thought. Nor does it deny the reality of our experience, but it is the phenomenal world which constitutes the whole of that experience. Knowledge is knowledge of how the world appears to us. The noumenal world is not LIKE anything; it is only LIKE something to be a consciousness IN the world2. Things as they are in themselves have no qualia (the sensory properties of our perceptions, like the redness of red, or the hardness of hard).
This is true up to a point, I think, but it could be argued that in concentrating on perception Kant missed the fact that we also ACT in the world; if this is to be more than mere illusion the noumenal world must also have its own organising principles, and perceptions/actions must be TRANSLATIONS from one framework to the other; in order for this to be the case both the phenomenal world and the noumenal world must share a single self-consistent set of rules.
Kant and the Self
According to Descartes the self was a thinking thing, quite different from the extended material body. The primary objection to this Cartesian dualism was that there was no explanation of how the two could interact.
David Hume, in examining his own awareness, declared that he could not find a self, but always found that his awareness was of something; he came to the conclusion that his self was nothing other than the sum total of these impressions. This was the bundle theory of the self.
Kant, however, came to the conclusion that the self, the "I think" that accompanies all our awareness, was a construct of the activity of the mind in bringing together its sensations amd cognitions. He called this the transcendental unity of apperception.
In keeping with his basic metaphysics he also recognised that the self which we perceive was necessarily a phenomenon only, existing in space and time, and beyond this there must be a noumenal self, whose operations were entirely unknown to us. Although we are not dealing with Kant's ethical system here it is worth noting that this concept is central to it, providing an explanation of the possibility of free will outside the deterministic rules of cause and effect operating in the phenomenal world.