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.
Narrated by Aidan Turner. Scripted by Nigel Warburton.
From the BBC Radio 4 series about life's big questions - A History of Ideas.
This project is from the BBC in partnership with The Open University, the animations were created by Cognitive.
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