In this letter to Follesdal, Lewis presents an early draft of his famous theory of counterparts.
Do Possible Worlds Exist?
Famously, Lewis believed in the existence of infinitely many physical possible universes. Lewis had been an early adopter of modal logic. Such logics usually explain ‘necessarily’ as ‘true in all possible worlds’ and ‘possibly’ as ‘true in some possible world’. But Lewis was also a devoted student of Quine. Quine stressed we should only believe in things we can’t avoid talking about (as logicians say, ‘quantifying over’) in our best theory. And we should only believe in things once we’re sure what they are and aren’t identical with (‘no entity without identity’). Quine urged that there were no good criteria for deciding when things in two possible worlds were identical. In our January 2017 letter of the month, Lewis explained why he believed in physical possible worlds. He thought that he could describe exactly when two things in possible worlds were identical: never. Possible worlds don’t share any parts.
When we talk about what possibly could have happened to ourselves in other possible scenarios, we are not just talking about ourselves. We talk about our counterparts, the people most like us in alternative possible worlds. It is true that I might have been a surgeon if the person most like me in some relevant possible world rather like this one is a surgeon. In this letter, Lewis lays out a rigorous logical treatment of counterpart theory.
Quine and Lewis
Some contemporary philosophers think of Lewis as a philosopher with a robust sense of reality, and of Quine as opposed to metaphysics. But the letter linked above shows a clear influence upon Lewis of Quine, his PhD supervisor. A recent paper of mine argues that the development of counterpart theory is also influenced by Quine’s idea that essence is relative to context. We will develop these ideas further in the project monograph.
You can read the paper here: https://doi.org/10.1093/monist/onx007
Or here: https://www.academia.edu/29979058/The_Quinean_Roots_of_Lewiss_Humeanism_published_in_The_Monist
Reference: Frederique Janssen-Lauret (2017) `The Quinean Roots of Lewis’s Humeanism’, The Monist, 100(2), pp. 249-265.