David Livingstone Smith at Philosophy Talk:
Philosophers are as much concerned about the possibilities that lie ahead as anyone else is. But philosophers are also interested in possibilities for a different reason—or rather, in a different way. When we consider possibilities, most of us are curious about what is possible, but a lot of philosophers are also curious about what possibility is. Put a little differently, a lot of philosophers are interested in the question of what it is that we’re talking about when we talk about possibility.
You might respond, “Well it’s obvious, isn’t it? When I say that it’s possible that it’s going to snow in Savannah tomorrow, I’m just talking about the fact that it might snow in Savannah tomorrow.” But this doesn’t really get us anywhere, because it’s just using different words to say the very same thing. It’s like saying “Wealthy people have lots of money.” Duh.
There’s been a whole industry in philosophy devoted to solving the puzzle of possibility. To describe this fully would require a book rather than a blog, so I’m going to confine myself to sketch just a tiny bit of it. There are plenty of great sources for those who want to learn more.
The most notorious, and most influential, way to address the problem of possibility was the brainchild of the philosopher David Lewis, and it’s really, really weird. The idea is that true counterfactual statements aren’t really counter to the facts because if that were the case there’s no way that they could be true. So there simply must be facts that make these statements true. These are obviously not facts about our world (it didn’t snow in Savannah today), so they must be facts about other worlds—parallel universes or, in the philosophical jargon “possible worlds.” When I truly say it could have snowed in Savannah, this is true because there’s some world—some parallel universe—where it snowed in Savannah.
More here.