A history of ideas. Presented by Melvyn Bragg but told in many voices. Each week Melvyn is joined by four guests with different backgrounds to discuss a really big question. This week he's asking 'How can I know anything at all?' Helping him answer it are physicist Tara Shears, lawyer Harry Potter, philosopher Clare Carlisle and neuropsychologist Paul Broks. For the rest of the week Tara, Harry, Clare and Paul will take us further into the history of this idea with programmes of their own. Between them they will examine: David Hume's debunking of miracles; Wittgenstein's attempt to prove that other people have minds; Karl Popper's idea of falsification, which underpins the scientific method; and George Berkeley's approach to a famous philosophical problem - If a tree falls in a forest and no one is around to hear it, does it make a sound? Producer: Melvin Rickarby.