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 every other possible world is as real and concrete a fact as this actual world is, and that they appear to be less so only because of some word magic -- "actuality" is indexical.
What is "indexical"? Let's chase the rabbit of definitions a bit further. A concept is indexical if it depends upon where the speaking is standing. Thus, "here" is indexical. I call my friend, at whose home I was recently a guest, and say, "Oh my gosh, I've lost my watch!" He says, "Relax, it's here."
I know the meaning of the word "here," so I know that my watch is at my friend's home. It isn't at MY "here"!
Likewise, "now" is indexical. If I say at the right moment "the sun is now rising" it'll be true. If I wait too long to say it, it'll be false.
Likewise of course the pronoun "I" is indexical.
Lewis's defining idea is that the difference between the actual and any possible world is merely indexical. The world in which goats (like the fellow portrayed above) talk, and for that matter speak fluent French, is as real as this one, it is simply not the one that I am in.
Now you see, I hope, why the pro-metaphysics surge was necessary to lift his name up into our top-thirty.
It is fun to think about, but extreme modal realism has not caught on. It has become more of a lighthouse warning of the rocks than a welcoming beacon.
Why did Lewis believe in modal realism? We'll save that for another day.