Immanuel Kant

Immanuel Kant spent his whole life in Königsberg, never travelling more than a few miles from it, and lectured for decades before publishing, at fifty-seven, the Critique of Pure Reason. It asked what the mind contributes to experience and concluded that space, time and causation are the forms of our knowing rather than things in themselves. Two further critiques did the same for morality and for beauty, and the Groundwork stated the moral law that every rational being gives itself. He is the hardest author on most lists and the one after whom philosophy could not go back.