Melvyn Bragg and his guests discuss the work of George Berkeley, an Anglican bishop who was one of the most important philosophers of the eighteenth century. Bishop Berkeley believed that objects only truly exist in the mind of somebody who perceives them - an idea he called immaterialism. His interests and writing ranged widely, from the science of optics to religion and the medicinal benefits of tar water. His work on the nature of perception was a spur to many later thinkers, including David Hume and Immanuel Kant. The clarity of Berkeley's writing, and his ability to pose a profound problem in an easily understood form, has made him one of the most admired early modern thinkers. With: Peter Millican Gilbert Ryle Fellow and Professor of Philosophy at Hertford College, Oxford Tom Stoneham Professor of Philosophy at the University of York Michela Massimi Senior Lecturer in Philosophy of Science at the University of Edinburgh. Producer: Thomas Morris.