The idea of an infinite number of worlds - each containing all the possibilities presented in our own world - might initially seem bizarre. Certainly David K. Lewis had a hard time convincing his fellow philosophers of his theory. But its application to theories of possibility - and its undeniable similarity to how we already think about time - means that we must make sense of its extraordinary claims, writes Daniel Nolan.

David Lewis (1941-2001) was an influential American philosopher of the second half of the twentieth century. His biggest impact in philosophy has been in the field of metaphysics.