One of the Big 30 of philosophy in the list I presented yesterday, perhaps the least well known of them to non-philosophers, is D.K. Lewis. More fully, that is David Kellogg Lewis (1941-2001). He is hanging on at the end of the list, #30, but he is there, in the elite company, nonetheless. He got there, interestingly, because of a late surge in what one may call pro-metaphysics votes in the polling. The earlier votes had had more of a positivist slant to them. This late pro-metaphysics surge not only got Lewis onto the list it put Aristotle at the top and moved Hegel up (past Nietzsche) to #17. So let's talk a little about Lewis's defining idea. He is considered the outstanding exponent of extreme modal realism. Let's break that down: (1) modal logic is the branch of logic that addresses issues of possibility and necessity; (2) modal realism is the view that what are called "possible worlds" are real facts; Lewis' extreme modal realism is the view that...