- Let's fix some terminology at the start. A world (or possible world – for me, the 'possible' is redundant) is,
There is at least one world, the world we are part of. It is an actual world, the actual world if there are no "island universes." Worlds that are not actual (if any) are merely possible. A realist about possible worlds believes that there is a plenitudinous plurality of worlds: whenever something is possible – for example, that donkeys talk, or that pigs fly – there is a world in which it is true.
- first, an individual, not a set or class;
- second, a particular, not a property or universal;
- third, concrete in this sense: it is fully determinate in all qualitative respects; and,
- fourth, a maximal interrelated whole: each world is internally unified, and isolated from every other world.
- There is more than one way to be a realist about possible worlds. Realists divide into two camps depending upon their account of actuality. According to David Lewis, the worlds are ontologically all on a par; the actual and the merely possible differ, not absolutely, but in how they are related to us. Call this Lewisian realism. Most philosophers grant that Lewisian realism, if true, would bring substantial theoretical benefits to systematic philosophy. Nonetheless, few philosophers have been willing or able to believe it. Often the obstacle to belief is the supposed ontological extravagance that accompanies any full-blown realism about possible worlds: belief in talking donkeys and flying pigs – even if they are spatiotemporally and causally isolated from us – is deemed simply preposterous. But that objection is based on prejudice, not argument; and it is not a prejudice I share.
- Objections to Lewis's account of actuality, however, are another matter. I take it to be conceptually evident that actuality is absolute, not relative, and that, moreover, the distinction between the actual and the merely possible is a distinction in ontological status: whatever is ontologically of the same fundamental kind as something actual is itself actual. When Lewis insists, then, that all worlds are ontologically on a par, I can only understand this – his protests notwithstanding – as saying that all worlds are equally actual. But that makes Lewis's defense of a plurality of worlds incoherent. For there could be no good a priori reasons for believing in a plurality of actual concrete worlds. And an analysis of modal operators as quantifiers over concrete parts of actuality, no matter how extensive actuality may be, is surely mistaken. I thus reject Lewisian realism.
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