In 1953, a posthumous work by the Austrian thinker, Ludwig Wittgenstein, revolutionised philosophy. Written in a unique, sometimes poetic style, Philosophical Investigations contained new thinking about the nature of language.
Simon Watts brings together BBC archive recordings of Wittgenstein's friends and colleagues, and hears from the philosopher's biographer, Ray Monk.
(Image: Ludwig Wittgenstein. Credit: Hulton Archive/Getty Images)