A Privatdozent drew no salary. From 1755 to 1770 he lectured at Königsberg on those terms, paid directly by whoever showed up, teaching around twenty hours a week, including a course in physical geography describing places he never went to see. He was passed over for the chair of logic and metaphysics in 1758 and got it in 1770, at forty-six. Then he published almost nothing for eleven years.
The problem he was sitting on those eleven years was Newton's. Newton had described a universe in which every event follows from the one before it, without remainder, and for physics this was a triumph; for everything else it was a threat, because if matter runs on law, what happens to a promise kept, a duty refused, a person held to account? Philosophy had split into two camps, and both answered badly. One insisted that pure reasoning could reach God, the soul and the beginning of the world. The other, following David Hume, pointed out that nobody has ever observed one thing causing another — we see one event, then a second, and supply the connection out of habit. Science worked, and nobody could say why it was entitled to.
What he was doing in those silent years was reversing the question. Philosophy had asked how our knowledge manages to match the objects out there; Kant asked what must already be true of us for an object to appear to us at all. His answer is the claim he is remembered for, and it fits in a sentence: the order we find in nature is the order our own minds impose in order to have an experience at all. Space, time and cause are the form anything must take to reach us.
Hume's problem dissolves — causation is guaranteed, because nothing could count as an experience for us without it — and Kant paid the price openly. What we know is the world as it appears. The world as it is in itself lies outside what we can know, and in exactly that space, he argued, freedom and moral responsibility survive Newton untouched.
The Critique of Pure Reason appeared in 1781, when he was fifty-seven, after twelve years of thinking and four to five months of writing. It landed on silence. The first substantial notice, in Göttingen in 1782, dismissed it as a rehash of Berkeley, denying that the world exists. Kant wrote the shorter Prolegomena in 1783 to explain what he had actually said. What finally worked was someone else's doing: from 1786 Karl Leonhard Reinhold ran a series of public letters on the new philosophy in a popular literary journal, and within a few years German philosophy was arguing in Kant's vocabulary.
The ethics followed in 1785, and it is the part most readers meet first. Act only on a rule you could will every other person to act on; never treat a human being merely as a means. Not because God commands it, not because it turns out well, but because a rational being gives itself the law and cannot coherently exempt itself from it.
It cost him: after Religion within the Boundaries of Mere Reason in 1793, a cabinet order from Friedrich Wilhelm II forbade him the following year to write further on religion. He complied, and resumed after the king died in 1797. In between he published Toward Perpetual Peace, which argued that war ends through institutions and republican constitutions rather than through better men.
Two centuries on, scholars still disagree about what his central doctrine even claims — whether things in themselves are a second world, or the same world considered another way. Almost everything written in philosophy after him is a response to him.