The Evolution of Modern Metaphysics: Making Sense of Things (81 page)

For Lewis, each of these possible worlds is a spatio-temporally unified cosmos causally independent of each of the others. And the things that constitute it really do exist. Thus given that there
could have been
flying pigs, in other words given that there is at least one possible world in which there
are
flying pigs, then there really are flying pigs. True, there are no flying pigs in the actual world, a fact that we might naturally express by saying, ‘There are no flying pigs.’ But that is no embarrassment for Lewis. He points out that our talk of what there is or is not is often tacitly and quite legitimately restricted, in a way that the context makes clear. Someone who says, ‘There are no flying pigs,’ is naturally interpreted as making a claim about what there is in the actual world – just as someone who opens the fridge and
says, ‘There are no eggs,’ is naturally interpreted as making a claim about what there is in the fridge. The former claim is no more vitiated by all the flying pigs in other possible worlds than the latter claim is by all the eggs in other people’s fridges. (See
Plurality
, esp. §§1.1, 1.2, and 2.1; cf. also ‘Counterpart Theory’.)
¹⁹

Whatever we make of such modal realism, there are two respects in which it appears radically un-Quinean. First, it appears not even to make sense unless there is a distinction to be drawn between what is necessarily the case and what is contingently the case; but this is one of the distinctions that we saw Quine repudiate in §4 of the previous chapter. Second, such modal realism appears to be ontologically extravagant; but, as we saw in §7 of the previous chapter, Quine is keen to acknowledge the existence of as little as possible.

In fact, the offence against Quine is not as great in either case as it appears. As far as the first point is concerned, the mere claim that all these possible worlds and their constituents exist is not, by itself, under any direct threat from Quine’s attack on the necessary/contingent distinction. It needs to be supplemented with the claim that there is a determinate yes/no answer to the question whether any given statement is true with respect to any given world. The real offender, in other words, is the idea of
what is the case in
a possible world. There is scope for an unregenerate Quinean to accept the first claim – to accede to the existence of countless possible worlds beyond the actual world – but to deny the second – to deny that we have any effective way of delineating, for anything that is the case, the range of possible worlds beyond the actual world in which it remains the case. (Quine himself toys with just such a view in Quine (
1969e
), pp. 147ff.
²⁰
) Still, for reasons that we shall glimpse in due course, such a view would not be acceptable to Lewis. It might conform to his strict definition of modal realism, which only requires the first claim, but it would not satisfy most of the philosophical assertions that he makes on behalf of his own view, which require the second claim as well. So as far as the first point is concerned, the offence against Quine, though not as great as it appears, is still pretty great.

As far as the second point is concerned, the offence against Quine is really not great at all. Yes, Quine champions parsimony. He wants to acknowledge the existence of as little as possible.
²¹
But that means: as little as is
compatible with the satisfaction of various other desiderata. Quine himself, as we saw in §7 of the previous chapter, reluctantly acknowledges the existence of mathematical entities, because of what he sees as the indispensable work that mathematics does in natural science. Lewis likewise acknowledges the existence of possible worlds and their constituents because of what he sees as crucial work that appeal to them does in our attempt to make overall sense of things. (More on this later.) He would hold no brief for modal realism were it not for that.
Methodologically
, his procedure is utterly Quinean. There may of course be disagreement between him and Quine concerning whether appeal to all these possible entities does do the work that he (Lewis) thinks it does. But that is another matter, more pertinent to the first point. As far as the second point is concerned, Lewis is no less keen to avoid extravagance than Quine is.
²²

This is symptomatic of the fact, as I see it, that the similarities between Lewis and Quine are far more profound than the differences. Each is concerned with providing an account of the most general character of reality. Each sees the need for a continual trade-off between various conflicting desiderata in the process of settling on such an account: conformity to common sense may have to be sacrificed for simplicity; simplicity may have to be sacrificed for accuracy, or, come to that, for conformity to common sense; parsimony may have to be sacrificed for explanatory power; and so forth. Each recognizes that what benefits outweigh what costs is a matter of non-codifiable judgment. Each refuses to draw the conclusion that the correct account is anything other than that:
the correct account
, unique, true, objectively affirmable. Each sees his project as broadly scientific. More fully, each sees his project as the project of making general sense of things in the only way in which sense can ever really be made of anything, which is to say in accord with broadly scientific methods and principles (this last point being an expression of their shared naturalism).

I mentioned conformity to common sense. We have already seen Lewis’ attitude to this in the previous section. He regards conformity to common sense in precisely the way I have just been outlining: as a good, but not a supreme good, and therefore a good to be weighed against others. He is well aware that modal realism, according to which flying pigs really exist, is an infringement of common sense. He refers amusingly to the ‘incredulous stares’ with which his view tends to be greeted (
Counterfactuals
, p. 86; see further
Plurality
, §2.8). But sometimes an infringement of common sense is a reasonable price to pay for certain theoretical gains. It is common for natural scientists to pay this price. They tell us, for instance, that the earth
is continuously rotating, and that simultaneity is relative to a frame of reference. Some of them see fit to tell us that glass is a liquid. Lewis tells us that there are flying pigs. The reason, in each case, is the same: it makes for a better overall account of the general character of reality. ‘We ought to believe in other possible worlds and individuals,’ writes Lewis, ‘because systematic philosophy goes more smoothly in many ways if we do’ (‘Abstract’, p. 354).

It is unfortunately beyond the scope of this chapter to discuss the impressive list of ways in which Lewis is able to show that systematic philosophy does indeed go more smoothly if we accept his modal realism.
²³
One representative example that I shall mention, though still not discuss, is the use of modal apparatus in analyzing counterfactual conditionals, that is statements of the form, ‘If it had been the case that
p
, then it would have been the case that
q
.’ On Lewis’ account, a statement of this form is true if and only if, roughly, in the possible worlds most like the actual world in which the antecedent holds, the consequent holds too (
Counterfactuals
, passim, and
Plurality
, §1.3). As it happens, I do not myself believe that modal realism pays its way. But I am more concerned with what it shows us about Lewis’ attitude and approach to metaphysics than with whether or not we should accept it. This is what I shall pursue in the next section.

A final point for this section. Since Lewis holds possible worlds to be spatio-temporally unified cosmoses that are causally independent of one another, there are grounds for the complaint that he is not properly attuned to all that might have been the case. Some of what might have been the case seems not to be the case in any of his possible worlds. For example, there might have been no space and no time. Again, there might have been (indeed, some physicists claim that there actually are) spatio-temporally unified cosmoses that are causally independent of one another, which means that there are possible worlds that Lewis can see only as pluralities of possible worlds.
²⁴

Lewis responds to this objection by simply denying the possibilities in question. In the case of the possibility of causally independent cosmoses he confesses that he ‘would rather not’ (
Plurality
, p. 71). But doing so, he thinks, is another infringement of common sense worth incurring for an overall gain in simplicity (ibid., pp. 71–72). This may seem cavalier on Lewis’ part. But it makes sense in terms of his broad programme. After all, the alleged possibilities are not particularly relevant to the work that he takes his modal apparatus to perform. (The analysis of counterfactual conditionals is a pertinent case in point. This analysis is meant to apply, first
and foremost, to counterfactual conditionals that are of real concern to us, never mind what would have been the case had there been neither space nor time.) If, as I suspect, this is the dominant factor in Lewis’ preparedness to reject these possibilities, and if, as I have also been urging, the work that he takes his modal apparatus to perform is of a piece with the work performed by the theoretical apparatus in any branch of natural science, then we see once more how far Lewis epitomizes the naturalistic spirit of his philosophical time and place.

4. Concerns About Modal Realism. The Concerns Removed, but the Shortcomings of Lewis’ Metaphysics Thereby Revealed

Lewis trumpets the various ways in which modal realism aids the smooth running of systematic philosophy. But he also considers various ways in which it seems positively to hinder it. For it seems to exacerbate certain philosophical problems (problems, it should be said, that are bad enough anyway). Thus consider the problem of induction: the problem of accounting for the reasonableness of certain inferences from the observed to the unobserved, such as the inference that someone instinctively makes when she assumes that the dog heard barking in the distance has only one head. If Lewis is right, then there are countless possible worlds, no different in kind from the actual world, in which everything is exactly as it is in the actual world until a time when dogs routinely start sprouting extra heads. How come we are so confident that our world is not one of those? (People in those worlds, let us not forget, are every bit as confident that neither is theirs.) Or consider the problem of accounting for the unreasonableness of certain courses of action, such as the infliction of gratuitous suffering. If Lewis is right, then any decision not to inflict gratuitous suffering in this world only means that gratuitous suffering is inflicted in countless other possible worlds instead. Why should it be preferable for the suffering to occur there rather than here?

Lewis shows admirably, it seems to me, that these worries are grounded in confusion. In fact his modal realism has no bearing whatsoever on these philosophical problems. Here is what he says about the first:

As a modal realist, I have no more and no less reason than anyone else to give over groundless faith in inductive luck. I have the reason everyone has: it is possible, and possible in ever so many ways, that induction will deceive me. That reason is metaphysically neutral. It becomes no better and no worse if reformulated in accordance with one or another ontology of modality. (‘Anselm’, pp. 22–23)

Concerning the second problem, he fastens on variants of the commonsensical notion that the reasonableness or unreasonableness of what a person
does is sensitive to what gratuitous suffering that person actually inflicts or refrains from inflicting, in a way in which it could never be sensitive to what gratuitous suffering that person might have inflicted or refrained from inflicting (
Plurality
, §2.6).
²⁵

The tactic that Lewis uses in both cases is akin to the tactic that I tried to use in
Chapter 3
, §4, to defuse the idea that each of Leibniz’ possible worlds is the best by its own lights. The tactic is simply to stop thinking of possible worlds ‘geographically’, and to revert to a more homespun understanding of what it means to say that something might have been the case, must be the case, or is actually the case. True, the inhabitants of other possible worlds lead their own colourful lives, which bring them their own pleasures and pains, their own joys and sorrows; they even have their own (sometimes very alien) natural environments. But these can have no influence on us, nor ours on them. They are just ways things might have been.

I said that Lewis’ modal realism has no bearing on the philosophical problems under consideration. In this context, that has to be taken as a compliment, since the bearing it would have, if it had any, could only be adverse. Nonetheless, in a broader context the comment should give pause. Here I am harking back to an observation that I made in §7 of the Introduction: that unless metaphysics makes a difference, it has no point. If modal realism were a way of thinking about necessity, possibility, contingency, and the rest that did not subserve any of the uses to which we put these notions, let alone if it were a way of thinking about them that positively thwarted the uses to which we put them, that would be objection enough to it. Leibniz’ modal realism, granted the success of his system, subserves the use to which we put such notions in thinking about how an omniscient, omnipotent, perfectly good being could fail to prevent what seems to be (aptly enough, in the light of the example that we have just been considering) gratuitous suffering. What uses does Lewis’ modal realism subserve?

Well, that, of course, takes us back to the previous section. But this discussion acts as a forceful reminder of how much turns on the philosophical work that Lewis takes his modal apparatus to perform. Deny that and, as Lewis would be the first to concede, you take away all reason to accept his modal realism. You do not refute it. You merely make it redundant.