Ludwig Wittgenstein

Wittgenstein wrote two philosophies in one lifetime, and the second one demolished the first. The Tractatus Logico-Philosophicus, written in the trenches of World War I, argued that meaningful language pictures facts in the world; everything that can be said can be said clearly, and "whereof one cannot speak, thereof one must be silent." Having (he thought) solved every philosophical problem, he stopped doing philosophy and taught primary school in a remote Austrian village.

A decade later he returned to Cambridge convinced he had been wrong. Language is not a picture of facts; it is a kit of countless language-games — promising, ordering, joking, praying, doing arithmetic — each with its own rules. There is no single essence of language and no single essence of meaning. Philosophy's job is not to build theories but to dissolve the puzzles we create when we use words outside their home games. "Philosophy is a battle against the bewitchment of our intelligence by means of language."

He was, by every account, the most charismatic and impossible figure in 20th-century philosophy. His students treated him with religious devotion. He gave away his vast inherited fortune and lived in two-room cottages.