On the way out of last weeks meeting, Robert asked me about possible worlds. Todd, Lord Matt and I were using the notion of a possible world in the previous weeks discussion, but we did not say much about them. So here goes:
A possible world is a complete way the world might have been. Suppose you could have been a rock star. Then there is a possible world at which you are in fact a rock star. That part is easy, and gives us a way to think about what might have been. But what are possible worlds?
The first philosopher to use possible worlds was Leibniz, long ago. He used them in part to answer the problem of evil: This is "the best of all possible worlds" and hence a good God would naturally chose this one to being into existence. For Leibniz, possible worlds were ideas in the mind of God. Imagine Him going through each of the ways the world might have been. When he finally reaches the best of all of them, he chooses to make that one real. Philosophers call that "actualizing" or "instantiating" that world. So this is the only real world, but the others exist as ideas in the mind of God. When you say "I could have been a rock star" you are saying that there is an idea in the mind of God, and in that idea, you are a rock star. Unfortunately, that world was not the best of all possible worlds, and hence you are actually stuck with only dreams.
Possible worlds lay dormant as a philosophical tool until revived by Saul Kripke in the 1950's and 60's. In order to deal with a difficult problem in Modal Logic (defining validity, and hence providing a semantics, for those curious), he reintroduced the notion of a possible world. Pressed later on their metaphysical stauts, Kripke said that a possible world simply was a counterfactual situation. A counterfactual is a conditional (if-then claim) where the antecedent is false. Example: If Germany had won the war, blah blah blah. That stipulates a possible world where Germany did win the war. On his view, these do not exist as ideas in the mind of God, or any where else. However, if possible worlds do not exist, in what way do they make modal claims true?
Enter David Lewis. Lewis (and I'm not making this up) was reading a work of science fiction in which someone creates a new invention which allows people to travel to other possible worlds. Inspired by this, Lewis defends what he calls "Modal Realism," the view that other possible worlds exist exactly like this one, just in a different space-time. Real people, real situations. What happens in those other worlds makes our claim about what might have been true. Our world is the actual world for us, but our world is a possible world for them, and their world is their actual world. For anything that you think might have been, there is a world, which exists just like this one, where that actually happened.
Actualism is the view that the only thing that exists is the actual world, and actualists reject Lewis's views. More next week, if there is any interest.