On the Plurality of Worlds

A defence of modal realism, developed from Lewis's John Locke Lectures at Oxford. Lewis holds that there are countless possible worlds, each as concrete as ours and spatio-temporally isolated from every other, and that our world is just one of them. 'Actual' is an indexical word, like 'here': it picks out the world the speaker is in. Chapter 1 argues that this hypothesis earns its place by its usefulness: it gives clear analyses of possibility and necessity, counterfactuals, the content of belief, and properties, which Lewis treats as sets of possible individuals. Chapter 2 answers objections, including the most common one, which he calls the incredulous stare. Chapter 3 criticises ersatz modal realism, the attempt to replace concrete worlds with abstract substitutes such as sets of sentences, which he divides into linguistic, pictorial and magical versions. Chapter 4 explains counterpart theory: things exist in only one world, and what is possible for them depends on their counterparts in others.

It is the boldest and most carefully argued account of possible worlds, and anyone working on modality, counterfactuals or metaphysics has to reckon with it.