What is modal realism?

Modal realism is a philosophical theory primarily associated with David Lewis. It suggests that possible worlds are just as real as our actual world. According to Lewis, to discuss possible worlds as merely useful fictions or as stories isn't sufficient. Instead, he proposed that for us to talk about possible worlds with intellectual integrity, we must consider them as complete and consistent entities. Every possible world is a complete description of a way that reality could be, and they are all equally real, even if we cannot physically access them 1 2.

David Deutsch adds an intriguing perspective by considering the implications of modal realism in physics. He discusses the continuity of the self in an infinite universe, questioning the physical existence of other versions of oneself across these possible worlds. Deutsch is open to the existence of nonphysical entities, like natural numbers, but remains skeptical about the physical existence of all possible worlds as posited by modal realism 3.

For those interested in deeper exploration of these ideas, the Conversations with Tyler and Mindscape Podcast offer episodes addressing modal realism and its implications.