If a tree falls in a forest and nobody is there to hear it, does it make a sound? That's the kind of head-scratching question that's popularly believed to occupy the time and brains of philosophers. It relates to the ideas of immaterialism proposed by Bishop George Berkeley who asserted that the only things that exist are minds and ideas in those minds. He said that matter didn't really exist and that, in any case, it was unnecessary to complicate things with such a concept. For Berkeley, "to be perceived is to be". But what happens to "things" when they are not being perceived? Did Bishop Berkeley really believe that his bed disappeared when he gets up in the morning and left the room? The answer is no, because there is the over-arching mind of God and God is always perceiving all things even when we are not. When Berkeley leaves the room God is still perceiving the bed so it doesn't pop out of existence. To try and get to grips with this Clare Carlisle talks to Dr John Callanan, a lecturer in philosophy from Kings College London and hears a neat limerick on the subject by Robert Knox. She also talks to the filmmaker Carol Morley whose documentary, Dreams of a Life, explored the story of a 38 year old woman, Joyce Vincent, whose body was found in her flat amongst half wrapped Christmas presents, the tv switched on. She had been dead for 3 years and nobody had noticed she wasn't there. The reader is Peter Marinker. Producer: Natalie Steed.