Modal logic deals with modalities, such as possibility, contingency, and necessity. A proposition is said to be necessary if it is true at all possible worlds. An example is "all bachelors are unmarried." A proposition is possible if it is true at at least one possible world, irrespective of its truth in the actual world. An example is "Unicorns exist." A contingent(ly true) proposition is true at the actual world, but false at some possible worlds. An example is "George Bush was elected president." Possible worlds are useful for explaining possibility and necessity, counterfactuals, essentiality and contingency, and other reasons, but the question remains, what, exactly, are they? The position of David Lewis is that possible worlds are as concrete as the actual world, but are spatiotemporally and causally isolated from it. The word "actual" is indexical. Its employment means that the world is the one that the speaker is at. So, our world is actual because it is our's, and other worlds are also actual for their members. There is a possible world corresponding to every way things could possibly be. In other words, there is a historical sequence identical to ours until it diverges at the time of the event specified by a counterfactual. So, there is a world in which everything happened exactly like in this one up until the point that I didn't type this message, afterwards, it differs. The reason for this postulation is its utility. Clearly, though, it is an extravagant claim contary to common sense, so other philosophers wish to make use of possible worlds without accepting that they are as concrete and real as the actual world. Lewis refers to these views as "ersatzism." His discussion of opposing views is fairly technical and lengthy, so I will not go into too much detail in this post. One of his arguments is that there could be an object distinct from any that is actual. If possible worlds are abstractions(i.e., maximal, consistent sets of sentences), then they are limited to the actual world. The problem is not dealing with a property or object that is alien because, after all, I am speaking of alien properties right now. It is rather that I am conflating them. When I speak of an alien property, I am unable to specify(for obvious reasons), so linguistic ersatzism is limited to far fewer possible worlds containing alien properties than realism.
Literature relevant to modal realism: On the Plurality of Worlds by David Lewis.
Literature relevant to modal realism: On the Plurality of Worlds by David Lewis.
