- The philosophical benefits that possible worlds offer are rich indeed. Everyday modal statements such as 'there are many different ways the world could have been' can be taken at face value, as talking about possible worlds other than the actual one. A wide range of concepts can be analyzed in terms of possible worlds in a logic that is familiar and well understood. Problematic and capricious de re modal statements can be tamed and understood. Physical necessity, deontic obligation, and other strengths of modality can be given unifying analyses in terms of possible worlds. Previously unanswerable questions about modal validity can be resolved. Once obscure intensional logics can be given a possible worlds model theory, and completeness and soundness results will have genuine philosophical significance. Unifying ontological reductions - propositions as sets of possible worlds, properties as sets of possible individuals - become available once we help ourselves to possible worlds.
- Though not conclusive, the fact that possible worlds enable us to unify and simplify our theories in such ways speaks in their favor. True, one can wonder whether the simplification and systematization that result are reason for thinking the resultant theory more likely to be true; harshly stated, such criteria can appear to be aesthetic rather than rationally compelling. But appeals to simplicity and unification are not restricted to philosophical theories; they appear in the theoretical sciences and in certain parts of common sense. When one doubts whether such theoretical virtues are worth having, one runs the risk of thereby being skeptical about a great deal more than just possible worlds.
- For all this, it is hard to accept David Lewis's view of possible worlds. On Lewis's view, merely possible worlds are like the actual one, concrete island universes containing - well, just about anything you care to think of, really. If it's possible, it'll literally be part of one of Lewis's possible worlds. The view that there is an infinite number of concrete island universes containing talking donkeys and stalking centaurs sits uneasily with common sense. The view that every possible object exists is an appalling violation. Though the kinds of simplicity and unification that possible worlds bring to our theories may be theoretical virtues worth having, simplicity of ontology, of the entities that a theory postulates, is a theoretical virtue too. Here, Lewis's theory of possible worlds scores very badly. One could grant the entire case for possible worlds' theoretical utility, grant that the theoretical benefits are great, yet still rationally believe that they are simply not worth the massive ontological costs.
- If only there were a way of getting all, or most, of the theoretical benefits that possible worlds have to offer without the excessive ontological costs and appalling violation of our common-sense beliefs, the case for possible worlds might be restored. Enter the ersatzer. Like Lewis, the ersatzer is a realist about possible worlds: possible worlds exist, can be referred to and quantified over in our theories and analyses. But the ersatz possible worlds have quite a different nature from Lewis's possible worlds. There may be no ontological free lunch - perhaps the ersatzer will have to invoke unreduced propositions, or states of affairs, or properties - but the ersatzer hopes his theory will be ontologically a lot cheaper than Lewis's and a whole load easier to believe in. The ersatzer's theory may not yield all the benefits that Lewis's theory offers (primitive modal concepts, for example, are hard to eliminate altogether on the ersatzer's scheme) but the ersatzer's laudable aim is to construct an ontologically parsimonious theory of possible worlds capable of getting as many of the theoretical benefits as possible.
- It is essentially this goal, rather than any particular thesis about the nature of possible worlds, that unifies the ersatzers. We cannot characterize the ersatzer as one who believes that worlds are abstract rather than concrete, as there are versions of ersatzism where possible worlds come out concrete. We cannot characterize the ersatzer as one who rejects mere possibilia - things that don't actually exist but that could have - for there's no reason why the ersatzer couldn't have ersatz possibilia along with his ersatz possible worlds. Ersatzism is better seen as a program rather than a particular unified position in the philosophy of possible worlds, and there are a number of different versions on the market.
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