Possible Worlds I: Modal Realism

Journal of Philosophical Logic, 2012

Among the most remarkable developments in metaphysics since the 1950's is the explosion of philosophical interest in possible worlds. This paper proposes an explanation of what possible worlds are, and argues that this proposal, the interpreted models conception, should be attractive to anyone who thinks that modal facts are primitive, and so not to be explained in terms of some non-modal notion of "possible world." I articulate three constraints on any acceptable primitivist explanation of the nature of possible worlds, and show that the interpreted models conception meets the three constraints. there were and what they were like. 1 Providing a non-modal explanation of the notion of being a possible world has occupied center stage in the thought of these philosophers. It will not, however, occupy center stage here. We are not going to require here that the notion of being a possible world be explained in thoroughly non-modal terms. Suppose, then, that modal primitivism is true: some modal facts cannot be reduced to, or even explained in terms of, any congeries of exclusively non-modal facts. Even granting that (SC) does not provide fodder for an explanation of modal facts in non-modal terms, there remain a number of alternative views of (SC)'s significance. One could suggest, for instance, that instances of (SC) are simply meaningless, since they all crucially employ the notion of being a possible world, and that notion has not been explained. Alternatively, one could simply deny that there are any possible worlds. Then there will be instances of (SC)-those involving true possibility claims-that are false. If we take either of these skeptical lines, then much of contemporary modal metaphysics will turn out to be either meaningless or a pack of falsehoods. And other disciplines that make liberal use of possible worlds, e.g. some strands of linguistics, ethics, epistemology, and decision theory, will suffer from the same defect. 2 This need not imply that talk of possible worlds in all of these

auslegung: a journal of philosophy

Mind, 2014

2020

The aim of the present book is to argue that even impossible things happen. By taking David Lewis’s Modal realism (hen- ceforth as “Mr”) seriously, i claim that besides infinitely many concrete ways things could have been, there exist ways things could not have been. i call them concrete impossible worlds.

Studies in History and Philosophy of Science, 2012

David Lewis was famous (among other things) for the meta-philosophical claim that knock-down refutations are rare to non-existent in philosophy. You can argue that the costs of a theory exceed the intellectual benefits but it is difficult, if not impossible, to prove that it is false. When it comes to choosing between philosophical theories, it is generally a matter of 'the price is right', and when it comes to refuting a theory the best you can generally do is to drive up the costs whilst diminishing the benefits. There may come a time when the costs are so high and the benefits are so low that the theory can't attract any takers, but this is usually due to a cumulative process of philosophical debate not to a sudden drop in net value brought about by a single decisive counterargument. When philosophers win, it's nearly always a victory on points and almost never a knockout. At the same time, Lewis subscribed to a philosophical thesis so outrageous as to cry out for knock-down refutation, namely that in order to make sense of modal truths, we must postulate an infinity of possible universes, all just as real as the one that we inhabit, but cut off from us in space and time. (This has the corollary that in order to make sense of causality we must postulate an infinity of items that have no causal connection to anything that happens in this universe.) Thus the temptation is to go for the double-whammy, the knock-down refutation of modal realism which is also a counterexample to the metaphilosophical claim. This was our aim when we first drafted this paper, but we can't honestly claim the double victory that we originally hoped for. We may have given Lewis's modal realism a drubbing but we don't pretend to have decked it. Even if we have succeeded in driving up the costs of Lewis's modal realism, it still retains enough benefits to attract some (rational) takers. Thus whatever the fate of Lewis's philosophical thesis, the meta-philosophical claim remains intact. ! We first wrote this article nearly twenty years ago when Lewis himself was very much alive, but for one reason or another we never got around to publishing it. Rather than rewriting it to take account of the sad fact of his death we have decided to leave it in the present tense. We dedicate this paper not only to our friend and colleague, Colin Cheyne (who does not think we should believe in entities which cannot causally affect us), but to the memory of David Lewis, a great philosopher and a great teacher. 2. Modal Realism: Mad Dogs, Milksops and Primitives In his metaphysical summa of 1986, The Plurality of Worlds, (henceforward POW) David Lewis famously defends a doctrine he calls modal realism. The idea is that possible worlds are real, indeed really real. They are concrete entities just like our own world, but cut off from us in space and time. To every logically possible setup there corresponds such a world. Lewis calls this the Principle of Plenitude. In fact, Lewis identifies his worlds with such setups a move which makes it difficult to specify the Principle of Plenitude in a non-circular way (POW, pp. 86-92). Lycan (1988) calls this doctrine 'mad dog' modal realism' 'Rape-and-Loot modal realism' and even 'Nuclear Holocaust modal realism', This is in contrast to moderate, milksop or, as Lewis describes it, ersatz modal realism. Ersatz realists believe in possible worlds all right. It is just that their possible worlds are less ontologically outrageous. Usually, they construct them out of actual entities of some kind: points, sets, sentences or what have you. This is 'paradise on the cheap', since we get all the theoretical advantages of possible worlds without the ontological drawbacks. ! Lewis thinks he can do without modal primitives and that the milksop defenders of ersatz worlds cannot. This he holds to be the drawcard of an otherwise unpalatable theory. His possible worlds are admittedly implausible things (especially in such abundance) and the chief reason for believing in them, rather than in ersatz possibilia of some kind, is that they enable us to eliminate modal primitives. ! Although sometimes denounced as a Meinongian, indeed a relentless Meinongian (Lycan, 1979), Lewis is, in fact, a rather heterodox Quinean. His aim is to explicate the modal concepts in terms of first order logic plus an ontology of possibilia. He retains a Quinean ideology, that is, the conviction that the resources of first-order logic, quantifiers, predicates, truth-functional connectives and the possible assistance of truth and satisfaction, are adequate to describe reality. 1 But unlike Quine (who prefers to believe there are no such things 2) Lewis recognizes modal facts. To accommodate these facts within a Quinean ideology 3 , to explain the modalities in terms of quantification theory, Lewis has to invent more reality. The price of his ideological restraint is ontological inflation. He must posit an infinity of objects to quantify over. But it is important to realize that despite his ontological exuberance, what Lewis is putting forward is a reductive