What does being a 'realist' about possible worlds involve? Is this kind of realism defensible?
The notion of 'possible worlds' is one that chiefly arose in philosophy in the mid-20 th Century, in response to a number of difficulties with our concept of modality, and in response to a particular empiricist challenge to modality. The idea of 'possible worlds' allowed logicians and philosophers to give a clear sense of modal notions of necessity, probability and contingency, but it has been argued that there is an intuitive discomfort that arises when one genuinely considers the existence of other, fully real worlds as a means of understanding modality, and it is easy to see why this is so. The title question asks us to explore, then, what one is committed to when one professes to be a 'realist' with regard to possible worlds. We should begin by examining these initial problems with modality to which the notion of 'possible worlds' is a potential resolution, before examining specifically the 'realist' position and what it involves, for which we will use the theory of David Lewis as exemplar. I will conclude that whilst the 'incredulous stare' that Lewis recounts as the most common response to his theory is understandable, given the theory, it is difficult to formulate any decisive philosophical objection against which Lewis' realism is not defensible. In our daily lives we freely make use of ideas of necessity and possibility, and indeed in philosophical discourse generally such notions as these are frequently employed. The notions of necessity, possibility and contingency are modal notions, and as Loux suggests it is arguable that we have been 'naive1' in assuming that we have a firm grasp of modal notions, since in fact they do not adequately fit into the logical systems that have been devised for dealing with the relationship between propositions, and thus we have no firm or complete sense of what we are committed to when employing modal terms. The classic empiricist challenge asserts that our experience of the world gives us no indication that it has modal features, and that as such we have no warrant to use such concepts. Experience, in contrast to modalities, never reveals what is necessarily or possibly the case, only what is the case, and thus empiricists after this style would argue that the only necessity is 'verbal necessity' - such as in the case of the proposition 'there are no unmarried bachelors', which is necessarily true, by virtue of the meanings of the words involved. Loux goes on to outline a second challenge to our use of modal terms. To be a philosophically adequate language, he suggests, a body of discourse must be 'extensional', which is to say that
'the
substitution of its constituent terms by co-referential expressions does not alter the truth value of the sentence2' (by 'co-referential expressions', Loux means two singular terms are co-referential if they name or refer to the same thing). From this point, using our well-founded systems of logic, and rules of internal sentence structure, if a piece of discourse is consistently extensional we can map out exactly which sentences follow from which sentences, and thus what any particular proposition commits one to by entailment. It is possible to demonstrate by example how introducing modal notions into discourse risks converting 'an extensional context into an intensional context': the proposition (1) the tallest man in the world is taller than everybody else in the world 1 M Loux, Metaphysics, Routledge (1998), p177 2 Ibid, p178
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