Life of Johnson , edited by George Birckbeck (Hill, Bigelow, Brown, 1921), p. 545. From a philosophical point of view, Johnson’s reply is woefully inadequate (nowhere does Berkeley claim that the mental nature of rocks and stones entails that they stop being hard), even though it does not lack a certain rustic charm. For what Berkeley would have replied, see his

Knowledge and Three Dialogues , edited by Howard Robinson (Oxford University Press, 1996), p. 129. A more sophisticated variant of Johnson’s definition is given in chapter 4 of David Deutsch’s

The Fabric of Reality (Penguin, 1997).

The quotation from Philip K. Dick comes from his 'How to Build a Universe That Doesn’t Fall Apart Two Days Later’, in

The Shifting Realities of

Writings (Vintage, 1996), pp. 259-80.

An interesting exploration of how the world would develop if all human beings suddenly vanished is presented by Alan Weisman in

The World Without Us (Virgin Books, 2008).

The quotation from Locke’s

An Essay Concerning Human Understanding

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