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...