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 ideas and perceiving minds.
Narrated by Aidan Turner. Scripted by Nigel Warburton.