Abstract
In his metaphysical summa of 1986, The Plurality of Worlds, David Lewis famously defends a doctrine he calls ‘modal realism’, the idea that to account for the fact that some things are possible and some things are necessary we must postulate an infinity possible worlds, concrete entities like our own universe, but cut off from us in space and time. Possible worlds are required to account for the facts of modality without assuming that modality is primitive – that there are irreducibly modal facts. We argue that on one reading, Lewis’s theory licenses us to assume maverick possible worlds which spread through logical space gobbling up all the rest. Because they exclude alternatives, these worlds result in contradictions, since different spread worlds are incompatible with one another. Plainly Lewis’s theory must be amended to exclude these excluders. But, we maintain, this cannot be done without bringing in modal primitives. And once we admit modal primitives, bang goes the rationale for Lewis’s modal realism.
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- 1.
Lewis’s ideological allegiance to Quine is most nakedly displayed in the opening paragraphs of his (1968) ‘Counterpart Theory and Quantified Modal Logic’. See also his POW (1986, pp. 1–20), and especially (1986, p. 4). However, in the 1990s there was some degree of relaxation in Lewis’s ideological austerity. He came to accept the plural quantification theory of George Boolos. See Lewis (1991) Parts of Classes, especially (1991, pp. 62–71). But by adding the apparatus of plural quantification to his ideology, Lewis ceases to be a heterodox Quinean and becomes something close to an apostate. (Boolos is very explicitly an opponent of Quine. See his (1985, pp. 331–334).) We are inclined to think that this undermines the philosophical rationale for modal realism. After all, if we are allowed the ideological indulgence of plural quantification given the rather minimal ontological savings it brings in, why not allow us the further indulgence of modal primitives given that the ontological savings would be massive?
- 2.
For an excellent survey of Quine’s views on modality and some sharp criticisms along the lines suggested see Haack (1978, ch. 10).
- 3.
See Quine (1961, p. 131).
- 4.
Lewis does not discuss ur-Plenitude explicitly. But his arguments against linguistic ersatzism, suggest that he would reject it as a fundamental principle for the reasons we have alleged. (The same goes for the principle of Consistent Describability discussed below.)
- 5.
This definition did not take a lot of extracting. On page 71 Lewis states that ‘things are worldmates iff they are spatiotemporally related’, and on page 69 that ‘a world is the mereological sum of all the possible individuals that are parts of it, and so are worldmates of one another’.
- 6.
David Armstrong proposes something similar in A Combinatorial Theory of Possibility (1989). However, Armstrong’s combinatorial worlds are ways actual objects could be, not ways that their duplicates are.
- 7.
- 8.
What is the other half of the biconditional, (ii”) ☐(☐(∃x)(Gx) ⊃ (∃x)(☐Gx)), supposed to do? Why does Plantinga need this? Perhaps to ensure that it is the same entity that is God-like in every possible world. If different beings possessed the God-like properties in different possible worlds, none of them would be truly God. So Plantinga must establish not only that a God-like being exists in every possible world, but that the God-like being is God-like in every possible world. It would not do, for instance, if the ‘God’ of this world were the ‘Satan’ of another and vice versa. This is not a problem for Lewis. If he were willing to accept Plantinga’s premises (which, of course, he is not) it would follow automatically that the ‘God’ of this world (or better the God-like being) was ‘identical with’ the ‘Gods’ in all the other worlds in the rather attenuated sense that it would be a counterpart of all the others. (At least it would follow on the assumption that you can have at most one ‘God’ per world.) And that is all the trans-world identity there is. Lewis would agree with Plantinga that being a counterpart of x is not really being identical with x, but would try to fob him off with the assurance that there is nothing more to be had.
- 9.
See for example Mackie (1982, pp. 55–63).
- 10.
- 11.
Originally published (1970).
- 12.
We had not noticed this passage until Lewis pointed it out to us. If our article performs no other function it will enable readers to make sense of Lewis’s rather compressed remarks on this topic.
- 13.
It is because they subscribe to this conditional that fictionalists about possible worlds are entitled to make use of them in elucidating the modal concepts. If your grasp of modal notions is a little shaky then the myth of possible worlds may be quite helpful. (‘It’s like this. Imagine there is a whole universe corresponding to every possibility …’) But such elucidations are not explanations. For a fictionalist, the modalities remain obstinately primitive.
- 14.
See for instance Lewis (1966, pp. 99–107).
- 15.
Our thanks to all those who have talked or corresponded with us about this matter, Mike Thrush, Jo Asscher, Ken Perszyck, Pavel Tichy, Denis Robinson, Richard Miller, Peter Milne, John Bigelow, Alvin Plantinga, Greg Restall, Chris Mortensen, Peter Forrest, David Armstrong and above all David Lewis. The paper was much improved as the result of a highly critical session at the 1993 AAP Conference in Adelaide attended by several of the above.
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Pigden, C.R., Entwisle, R.E.B. (2012). Spread Worlds, Plenitude and Modal Realism: A Problem for David Lewis. In: Maclaurin, J. (eds) Rationis Defensor. Studies in History and Philosophy of Science, vol 28. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-3983-3_12
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