Leibniz once stated that 'many stories, especially those called novels, may be regarded as possible, even if they do not take place in this particular sequence of the world which God has chosen.' That is, God's world did not include Don Quixote, although it could have done; it only included a certain Cervantes who 'invented' him and his adventures - indeed, invented him so vividly that he 'lives' on in readers' heads to the present. Similarly, God's London did not include Sherlock Holmes or Mrs Dalloway, nor could they have met, say, on Regent Street or in Kensington Gardens - being fictional, they could not have met anyone anywhere, except in the pages of a novel. In Leibnizian terms, it was not in God's plan to instantiate or actualise Don Quixote or Sherlock Holmes or Mrs Dalloway: they are possible entities only, whereas we ourselves and all the people who have ever lived or will ever live are not only possible but were, are, or will be, also actual.

For Leibniz God was, amongst other things, a sort of master-logician: he chose out of the infinity of possible worlds the one that was best. This was a claim that met an often sceptical response, most famously in Voltaire's Candide, in which Leibniz was caricatured as Doctor Pangloss. No one but a philosopher could think that we live in the best of all possible worlds - except for a theologian: for it is hard to see why a God who ...