Immanuel Kant (1724-1804) is widely recognised as one of the world's greatest and most important philosophers, but the lack, for over a hundred years, of an English translation of his major work, led to his being largely ignored by English-speaking philosophers until well into the 20th Century.
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 major work The Critique of Pure Reason, published in 1781, produced a theory of metaphysics which he regarded as a 'Copernican revolution1' in philosophy.
In the mid-18th Century, European philosophy was dominated by two competing schools of thought, Empiricism and Rationalism.
The Empiricists, notably the English philosopher John Locke (1632-1704), and the Scottish philosopher David Hume (1711-1776), held that the mind is a tabula rasa, a blank slate on which knowledge of the world is inscribed, and that our perceptions present 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 (1646-1715), 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 guaranteed?
Kant's resolution of this 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. These comprised the organisation of the understanding, through which the laws of mathematics, geometry and physics acquired a necessity that mere empirical observation could not give them.
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 observer, in the absence of the observer 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 seem to have thought. Nor does it deny the reality of our experience, but it is the phenomenal world that 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 in themselves have no qualia (the sensory properties of our perceptions like the redness of red or the hardness of hard).
It could be argued, however, that in concentrating on perception and thought Kant did not give due weight to the fact that we also act in the world; if it is to be more than mere illusion the noumenal world must have its own organising principles, and perceptions/actions must be translations from one framework to the other; in order for this to be so the noumenal world and the phenomenal world must share a single self-consistent set of rules.
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 and 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.