- David Lewis believes in the thesis known as modal1 realism. The thesis says that our actual world and its inhabitants do not exhaust everything that there is; there are other possible but non-actual worlds and their possible but non-actual inhabitants. Our world is but one of many possible worlds. There are as many possible worlds as there are ways the world could possibly be. Each world is isolated from any other in a rather strong sense; no two worlds overlap or are spatiotemporally or causally related in any way. To many this appears to be an outrageously implausible thesis. But Lewis has his reason for believing it. In a nutshell, he believes it "[b]ecause the hypothesis [of modal2 realism] is serviceable, and that is a reason to think that it is true." Serviceable for what purposes? For the purposes of philosophical analyses of various important notions. The more notions are analyzable by means of modal3 realism, the stronger Lewis' case for modal4 realism is. Lewis says of set theory as a metaphysical foundation of mathematics:
It offers an improvement in what Quine calls ideology, paid for in the coin of ontology. It's an offer you can't refuse. The price is right; the benefits in theoretical unity and economy are well worth the entities. (p. 4)
- He believes the same of modal5 realism. Among the analyses Lewis defends, some are cast in the mold of possible worlds directly, and others indirectly. The latter include analyses of causation6 and supervenience7. Those notions are analyzed in terms of other notions, which are themselves analyzed directly in terms of possible worlds. Directly analyzed notions include various modal8 notions, the notions of proposition, property, meaning, counterfactual conditional, attitudinal content, and verisimilitude. Let us focus on the directly analyzed notions. Lewis' argument may then be summarized as follows:
Understand those notions as involving quantification over possible worlds and their inhabitants, and you obtain the best (unified, elegant, and economical) analyses you have ever had. But to make sense of quantification over possibilia, you should believe in the existence of non-actual possible worlds and their inhabitants as constituting the universe of discourse for the variables to range over. Therefore, you should believe in the existence of such worlds and their inhabitants.
- This is an amibitious argument. It is not my concern in this paper to challenge it. Instead, I shall propose a natural extension of this argument and claim that if Lewis' argument is to be accepted, then the extended argument is also to be accepted. The extended argument is obtained from the original argument by replacing reference to possible worlds and their inhabitants with reference to possible and impossible worlds and their inhabitants. I shall first spell out what I take to be the virtues of Lewis' analyses of various notions in terms of possible worlds and their inhabitants, and then show that the corresponding analyses in terms of possible and impossible worlds and their inhabitants have higher degrees of those virtues than the original analyses. I shall conclude the paper with some speculations on the metaphysics of impossibilia. Throughout the paper I shall refer to the thesis that there are possible worlds in Lewis' sense but no impossible worlds, as 'the Lewisian modal9 realism,' and the thesis that there are possible worlds in Lewis' sense and also impossible worlds in an equally realistic sense, as 'the extended modal10 realism.' I shall reserve the term 'modal11 realism' for the thesis that there are possible worlds in Lewis' sense. Both the Lewisian modal12 realism and the extended modal13 realism are (versions of) modal14 realism.
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