Epistemology, or the theory of knowledge studies the nature of knowledge and deals with skepticism, the worry that nobody knows anything. Metaphysics concerns what exists, and what relationships hold between things that exist.Metaphysical issues include the problem of other minds, the mind-body problem, and the problem of personal identity. We can ask whether minds and bodies are distinct substances, and if so, how they are related. We may wonder whether machines can have minds. We can explore the conditions for personhood, and the issue of what makes any object the same object at two different times.The question of identity also arises when we think about modal notions, such as the notions of possibility and impossibility. We often distinguish between what is actual and what is possible. That you are now reading this text is actual. You are actually reading it. But you might not have. You might have read something else instead, or have done any of a number of other things. Philosophers sometimes talk about possibilities in terms of possible worlds. To say that you might have taken Geology 101 means that there’s a possible world in which you take Geology 101 instead of Philosophy 101. There’s also a possible world in which you take both, as long as they aren’t offered at the same time.
There are also possible worlds in which you don’t exist. I can easily imagine such worlds in my own case: My father applied to West Point when he was applying to college. He didn’t get in. But had he gotten in, he almost certainly would have been killed in Korea with the rest of the class he would have graduated with. So I can imagine the possible world in which my father got into West Point, went to Korea, and got killed. In that possible world I would not have had the opportunity to be born, and so the person I’m talking about would not have been my father! (So another way to say it is this: There’s a possible world in which my father isn’t my father.) So that’s a possible world in which I don’t exist. That’s just one possible world in which I don’t exist. There are countless others.
There are also possible worlds in which I do exist but where the world is different from the actual world. For example, there are possible worlds in which I exist, but where Occidental College doesn’t exist. In such a possible world I would not be a philosophy professor at Occidental College. I might be a professor somewhere else, or I might be a truck driver or front end alignment technician or a fisherman in Iceland. There are also possible worlds in which I exist, in which everything else that exists in the actual world also exists, but where some things are different. For example there is a possible world in which on Wednesday, March 7, 2007, everyone who, in the actual world is wearing shoes, is wearing sneakers, and everyone who is wearing sneakers in the actual world is wearing shoes in this other possible world.
There are also possible worlds containing objects and persons, and other entities that don’t exist in the actual world. Five headed philosophers are possible, but there aren’t any in the actual world. Because they are possible, there are possible worlds that contain them.
As metaphysicians, what should we say about possible worlds? What are they? Do they exist? Do the things in possible worlds exist? If so, how are they related to things that exist in the actual world? What’s the difference between existing in the actual world and existing in a possible world?
Ordinarily, we tend to think about possibilities in isolation. We might say something like: “It’s possible that it will rain tonight.” Now suppose that it does rain tonight. Then it’s raining tonight is part of what happens in the actual world. As part of the actual world, it is caused by other things in the actual world, and it has effects on other things, also in the actual world. Suppose the fact that it rains tonight means that the square dance in the quad is cancelled. Then, in the actual world the evening rain caused the cancellation of the square dance. It’s also possible that it won’t rain tonight. Still, supposing that it does rain, we can imagine what would happen if it didn’t rain. The dance might go ahead as planned. We can also think of further consequences: Ignat and Mathilda dance together for the first time, fall in love, and decide to spend the rest of their lives together.
Perhaps you’re getting the idea: Put a couple of possibilities together, think about the way they might be related to each other, and before you know it you have a complete description of everything that happens, and that’s a possible world. The philosopher most famous for thinking about possible worlds was Leibniz. Leibniz believed that God was the ultimate rational planner. It was up to God to create the world. That meant deciding what the actual world would be like. Since God is omniscient, on Leibniz’s view, God could easily conceive all the possible ways the world could be. God could think about whether it rains tonight or not, and whether it’s raining would or would not affect the meeting of Ignat and Mathilda. But that’s just a couple of possibilities. God thinks of them all, on this view, compares them, and then because God is completely good, he/she/it actuates the best of the possible worlds.
If there seems like there’s too much of an appeal to God for a philosophical argument in this last bit, the same point can be made without the theological baggage. We may not be able to think about all the possible ways the world could be, but the idea makes sense, and we can think of all the descriptions of all possibilities as the set of possible worlds. Whether or not the idea of a best of all possible worlds makes sense is a completely different issue, and one I’m happy to avoid for the present.
Some philosophers believe that all possible worlds exist. By saying this, they are not saying that all possible worlds are actual. Rather, they hold that the possible events, persons, and things in other possible worlds really exist as possibilities. Even if it doesn’t rain tonight, its raining was really possible, and so real, though not actualized. The possibility of its raining tonight exists. Five headed philosophers are really possible, no matter how unlikely. So they are real possibilities. That doesn’t mean they are likely, just that they are possible. Philosophers who believe that possible worlds are real are called modal realists.
Let’s see what a modal realist would say about those other possible worlds that include you. (I’ll use myself as the example, but you can easily reconstruct these examples using your own case.) In the actual world, I walk into class on Wednesday, March 7th with my backpack. But we can imagine my having forgotten my backpack. So there’s a possible world in which I walk into class on the 7th without my backpack. Now notice how I’ve described the situation: I said that there’s a possible world in which I walk into class without my backpack. If other possible worlds exist, then there appear to be two “me”s: the me who, in the actual world, walks into class as usual with the backpack, and the me who walks into class without the backpack. Which one is really me? The modal realist says that they are both me – they are me in different possible worlds. But how could this be? How could it be true of me that I walk into the room with my backpack and that I walk into the room without my backpack at the same time? This looks like a blatant contradiction.
The modal realist replies that it would only be a contradiction if I had the backpack and didn’t have the backpack in the same possible world. But in the actual world I simply have the backpack. Could the possible me who doesn’t have the backpack, the “me” in another possible world, – let’s call him my “counterpart” – cross over into the actual world and coexist with me in it? If that happened then the actual world and another possible world would overlap. That’s what the Twilight Zone episode, “Mirror Image” is all about. After you watch the episode, you should consider how the episode answers this question, and whether that answer is correct. Can two possible worlds overlap? Could you run into a counterpart in a bus station or anywhere else? Suppose it isn’t possible. But the “Mirror Image” depicts this situation. So “Mirror Image” depicts something that isn’t possible. But how can we tell a story that makes sense but is impossible? Suppose is is possible, and the episode depicts a real possibility. Then there is a possible world in which you exist and a possible you from another possible world (your counterpart) exist in a converged world. But then both you and your counterpart are actual, existing persons – but are they they same or different persons? It seems that they can’t be the same, but they can’t be different either.