Introduction

If possibilities are real, what follows? Lots of things. But, if we use possible worlds to explain the nature of possibility, does the reality of possibilities mean that possible worlds must be real?

Some argue for a “yes” answer. Famously, David Lewis was the chief of those in the twentieth century arguing for a “yes” answer. Perforce, we will spend a great deal of time in this chapter considering Lewis's ideas, but not in the detail they deserve. Lewis takes the possible worlds account of possibility and necessity, and some other modal notions such as ought, with utmost seriousness. An analogy has been drawn between his approach to possible worlds and Gallileo's ideas about the heliocentric solar system.

At the time of Gallileo there were two astronomical pictures of the solar system: Gallileo's picture and Ptolemy's picture. Ptolemy's picture had the Earth at the centre of the solar system, and everything orbited about the Earth. The vast majority of astronomers accepted the Ptolemaic account. The Ptolemaic picture was more in accord with everyday intuition, common sense and observation. There is an excellent picture-book account of Ptolemy's account that shows how closely it follows observation. It was not a “silly” account.

Gallileo's picture, by contrast, had the Sun at the centre of the solar system, and the planets, including Earth, orbited the Sun. His motto was, “The earth moves”. This solar-centric account is quite contrary to everyday, and every night, common-sense observation.