- Although the notions of necessity and possibility (the so-called "modal1 notions") seem indispensable in metaphysics, empiricists have traditionally challenged the appeal to these notions. Developments in the semantics of modal logic2 have, however, given philosophers reason to believe that the empiricist challenge can be met. At the core of modal3 semantics is the idea of a plurality of possible worlds. Metaphysicians have argued that this idea is perfectly respectable, indeed, that it is implicit in our pre-philosophical thinking about modal4 matters; and they have claimed that it provides the tools for clarifying not only the concept of de dicto modality5 (the notion of necessity or possibility as ascribed to a proposition), but also the notion of de re modality6 (the notion of a thing's exemplifying a property necessarily or contingently).
- There have, however, been two different ways of invoking the concept of a possible world. Some philosophers have thought the concept of a possible world provides the materials for a reductive nominalism. David Lewis's theory of modality7 represents the best example of this approach. Lewis takes the notion of a possible world as primitive and uses it to provide reductive accounts of the notions of a property, a proposition, de dicto modality8, and de re modality9. His is a technically elegant theory, but it requires us to construe all possible worlds as equally real and fully concrete entities, and most philosophers find that too high a price to pay for the elegance of the theory.
- Accordingly, many philosophers impressed with the power of the notion of a possible world endorse an alternative approach, one most fully developed in the work of Alvin Plantinga. On this view, the notion of a possible world is taken to be one element in a network of interrelated concepts including the notions of a property, a proposition, de dicto modality10, and de re modality11: and the claim is that while we cannot reduce any of these concepts to concepts outside the network, we can clarify the concepts in the network by showing their relationships to each other. Plantinga construes possible worlds in Platonistic fashion as maximally possible states of affairs and identifies the actual world as that maximally possible state of affairs that actually obtains, thereby endorsing the whole framework of possible worlds while holding onto a thoroughgoing actualism that insists that only what actually exists is real.
- Problems about modality12
- Possible worlds
- Possible worlds nominalism
- The metaphysics of possible worlds nominalism – David Lewis
- Actualism and possible worlds - Alvin Plantinga
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