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.

One of his views, above all else, struck many of his fellow philosophers as fantastic and hard to believe. Lewis held that, as well as our concrete universe, there also existed infinitely many variations, cut off from our "world" each in their own space and time. These "possible worlds" were not just variants with our fundamental laws of nature but different parameters, they included every way of reassembling the kinds of objects and properties found in our world, plus endless kinds objects and properties not even dreamt of by our physicists.