TY - JOUR
AU - Divers, John
AB - The interpretation of quantum mechanics due to Everett (1957) postulates the existence of many worlds. The analysis of modality due to Lewis (1986) is supported by the postulation of the existence of many worlds. Accordingly, if we take the modally relevant plurality of worlds (or multiverse) to be the one that this bit of physics gives us, we might obtain a unified and comprehensive naturalistic cosmology: a theory which accounts for all modal truth in terms of being, and which carries the epistemological authority of natural science. Thus the quantum modal realism (QMR) of Alastair Wilson in The Nature of Contingency. This is a thoroughly stimulating and clearly written work that argues for a genuinely big idea. Taking on both the philosophy of physics and the philosophy of modality is a big job, and I doubt that many share Wilson’s aptitude for the task. I should make clear that I am not competent to address the (philosophy of) physics side of the QMR equation. Accordingly, in what follows, I am taking what is granted to me about quantum mechanics, the Everettian interpretation thereof, decoherence, and so on. With respect to the physics, however, I should make explicit at the outset a crucial point. The Everett multiverse is usually, and informally, characterised as one in which the worlds split, branch or overlap (Ch.2: §2.1–2.3). However, Wilson argues (Ch.2: §2.4–2.7), this is another case in the quantum mechanical territory where the formalism, even at the level of the Everett interpretation itself, significantly underdetermines the metaphysics. In particular, Wilson argues: (a) we are at liberty to understand the Everett space as being constituted by mereologically disjoint worlds and (b) we ought to do exactly that, since that is where the greater (overall) theoretical utility lies. Insight (a) is striking. And while Wilson is extensive in citing sources in philosophy of physics on which he relies, there is little room for doubt that the insight is largely, if not entirely, his own contribution. I am minded, however, to quibble with the subsequent choice of terminology. For I find it jarring that the preferred description of the worlds in the disjoint version of Everett is as (their) ‘diverging’. That term (strongly) connotes, and I think not only to me, exactly what is not intended – that is, splitting, bifurcation, and so on. But putting that minor matter aside, I turn to the modal philosophy. Many convinced themselves long ago that there is something deeply wrong with the Lewisian philosophy of modality, indeed, that it is so deeply misconceived that it would be doomed even if we were to know that the ontological claims of genuine modal realism are true. That conviction is typically rooted in the prior conviction, whether argued (Shalkowski 1994) or simply asserted (Stalnaker 2012, p. 4), that modality must be, in one way or another, primitive. Wilson does not speak to the primitivist constituency, for his view is (Ch.1): (i) that Lewisian modal philosophy is good in the ways that Lewis takes it to be good (theoretical virtues) and (ii) that the (most) significant problems that it faces are those to which a quantum mechanical reconception is equipped to provide answers. How could we know the modal facts so understood and why should we be concerned with them so understood (Ch.2: §2.1-2.3)? I will (happily) operate within this reduction-tolerant philosophical frame of reference. That is a frame of reference which I take to be bounded on one of its sides by Lewis (of course) but on another – and crucially – by Quine. And to put up front my overarching concern with the project of The Nature of Contingency, it is not clear to me what QMR may be supposed to do for us, philosophically speaking, once its relationship to these defining positions is charted. I begin by speaking to a claim that Wilson seems keen to press but which both Lewis and Quine rule out of court. Following that I offer a Quinean perspective on QMR, a Lewisian perspective on QMR and finally (on the basis of all that) my meta-philosophical comment on the implications of QMR for metaphysical modality. It is puzzling that QMR is supposed to ‘underwrite’ a system of modal logic (p. 3, pp. 37–40). The logic in question is quantified S5. Many philosophers seem determined to find in Lewis (1968) a semantics for modal logic – despite Lewis’s explicit and up-front statement that this is exactly what is not on offer there (ibid. p. 113). And those philosophers may be puzzled that it is quantified S5 that is selected by Wilson’s counterpart-theoretic semantics. But there is a story (pp. 37–40), and which version of modal logic is ‘underwritten’ is not where the real puzzles lie here. The first puzzle is why, in light of his surrounding views, Wilson thinks that we need a modal logic at all. Neither Quine nor Lewis accepts that modal subject-matter requires a special logic. Neither, of course, finds any fault with the proof theory, the model-theoretic semantics or (even) with some intended interpretations of what we might call, with requisite neutrality, box-diamond logics. But the position of both Quine and Lewis (albeit, based on diverging rationales) is that all the logic that we need to apply to our modalizing is first-order non-modal logic. It is not at all obvious to me, then, why Wilson should not join them in this opinion. Presumably, Everettian quantum mechanics has a formulation that is free of modal operators. But if therein we have a theory that caters for all the truth about modality, why do we need to apply in our modalizing a logic which is for the very operators that are conspicuously absent from that theory? To bring the second puzzle properly into focus let us assume that the proponent of QMR has identified some need for, or advantage in, the deployment of a modal logic. How, then, is QMR supposed to ‘underwrite’ one particular modal logic rather than another? I find it hard to see past the possibility that a subtle distinction has been overlooked here. Even if all the truths about necessity (as well as contingency, and so on) are tracked by QMR, even if all the truths about necessity are ‘grounded’ in the QMR facts, and so on, that is categorically insufficient for what we need in order to characterize validity for modal inferences. For, validity is a matter of what is true in every model and when we are dealing with a modal logic, a model is a single (representation of a) multiverse. So validity, here, lies not with the multiverse (of the intended model) but with the plurality of multiverses that are represented across our range of admissible models. Now, it is an open secret that we get out of the models by way of validity whatever we choose to put into them by way of constraints on admissibility. And it is certainly the case that there is no technical obstacle to deeming admissible only models of Wilson’s preferred version of Everettian quantum mechanics. But why would we want to constrain our logic in that way? Everyone, I take it, sees the need for a first-order non-modal logic in which the admissibility of models is not constrained by (what we take to be) the true physics. But, if I have understood correctly, Wilson is committed to first-order non-modal logic being exactly so constrained since (a) a first-order non-modal logic is (unavoidably) given as a fragment of the quantified modal logic and (b) the model set for the quantified modal logic is constrained by the truths of physics. Indeed, on one reading of what is going on, the constraints on models are such that the (semantic) consistency of even branching Everettian QM, never mind any more remote prospects, is ruled out of court since such a theory has no model among those deemed admissible. But whether this apparent consequence really is a consequence, intended or otherwise, of the ‘underwriting’ should not distract us. The major issues here are why a proponent of QMR should want to select a modal logic at all and what kinds of considerations they should appeal to in constructing a semantic theory which is invoked in order to select it. Granted, the tilt at modal logic may be an eminently dispensable aspect of the dominant project. So, let us look beyond that episode to try to understand better what that dominant project is. It is natural to think that QMR might gladden the Quinean heart. And, indeed, Wilson claims a Quinean antecedence for his methodologically naturalist approach to metaphysics (pp. 17-21). But one might look further into that source than Wilson does. If QMR were to succeed, it would offer a great deal of what the Quinean desires in an explication or accommodation of modal discourse. For we would have, in QMR, a strong basis for the elimination of modality from those parts of our science that tell what much of traditional metaphysics hopes to tell: what our fundamental ontological and typological commitments are, what it is of which all things are composed and which laws encompass all of being. Thereafter, the familiar and persistent modal talk within the adjacent and supporting discourse of scientific communication (in journals, laboratories and lecture theatres) can be treated as relating to reality in the sorts of indeterminate and partial ways that the colour talk and the causal talk (that we also find in that discourse) does. And that is to say not in a way that directly introduces us to the fundamental elements of being. Moreover, Wilson moves decidedly in the Quinean direction by not constraining the modal truth that is reflected in QMR to match in extension any notional body of pre-theoretical opinion (more of which anon). So why, given the resources afforded by QMR should, we not go the whole hog and, as Plantinga (1987) puts it, just Quine modality? Part of the explanation of why Wilson does not own the Quinean potential of his project is that he is (clearly) not thinking of Quine’s modal philosophy in the same way as I am. Wilson and I can happily agree that ‘Quine’s notorious hostility to modality is overstated’ (p. 6). But thereafter Wilson and I part ways. Wilson’s Quine (p. 6) is the familiar character of contemporary philosophical lore whose concerns about modality were addressed – indeed, ‘assuaged’ and ‘soothed’ – by Kripke. Firstly, the model theory for quantified modal logic (where, I imagine, Wilson has in mind Kripke (1959 and/or 1963)) dealt with allegedly Quinean ‘fears about incoherence or inconsistency in the main logical systems’ (p. 6). Secondly, the informal or impure take on the model-theoretic semantics – the philosophically extended presentation as possible-worlds semantics in Kripke (1980) – provided a ‘compelling intuitive gloss on reasoning about possible individuals via possible worlds’ (p. 6). So, I can appreciate, if Wilson is thinking that the Quinean motivation for dispensing with modality in logic, and in other aspects of fundamental cosmological theory, was based entirely on considerations that Kripke undercut, he will be apt to think that he might safely proceed without taking the Quinean eliminative position into account. However, the Quine that I have in mind, as a potential appropriator of QMR, is not the familiar character of contemporary philosophical lore. My Quine, rather is the author of – inter alia – Quine (1981, pp.173-4) who explained both (a) why Kripke’s model-theoretic interpretation of quantified modal logic was irrelevant to the grounds of his (Quine’s) modal scepticism and (b) why Kripke’s informal possible-worlds semantics was little more than a repackaging of what, in metaphysical modality, he was sceptical about. My Quine objects to the inclusion of modalities in the metaphysically and epistemologically telling parts of science because they are complicating and dispensable elements. What, then, is the objection to executing, on the basis of QMR, the elimination sought by my Quine? Perhaps it is meta-philosophical. Perhaps, other things equal, we should reduce wherever we can rather than eliminate. And, of course, we have an important precedent for that approach in Lewis. So let us consider whether it is to Lewis – in this respect and others – rather than to Quine that the proponent of QMR ought to look in order to articulate the philosophical significance of the demodalized basis of modality. Lewis’s philosophy of modality is defined by two different, but intimately related, analytic projects (see Lewis (1986, 1992) and Divers and Fletcher (2020)). One project is the analysis of modal concepts: this is aligned with the project of interpretation and a semantic theory of truth-conditions which is supposed to apply to the sentences in the modal fragments of natural languages. The other project is the metaphysical project of accounting for modal truth in terms of being. The recent history of our subject teaches that those who are equipped only with the metaphysical hammer of truth-making come to see everything offered by Lewis as the truth-makers, for modal sentences, that they are determined to nail. Accordingly, much of the motivation, concern and discipline that emerges from Lewis’s interest in analysis of the non-metaphysical kind (concepts, meaning, and so on) gets left behind when the Lewisian goods are appropriated. In particular, what is often left behind is a fundamental principle of Lewis’s methodology: that the limits of possibility drawn by our metaphysics should be constrained, primarily, by what we need to best systematize pre-philosophical modal opinion. (Although, certainly, it is allowed that best systematisation might require us to recognize, and ontologically secure, a tad more or a tad less possibility than we are pre-philosophically disposed to endorse.) One manifestation of the application of that principle is that insofar as Lewis is inclined to offer a characterization of absolute modality it is as analytic (beginning with (Lewis 1968, p. 115)): it is not as metaphysical, and here I note that there are no entries for ‘metaphysical modality’ in the index of Lewis (1986). Indeed, he consistently held – for example, in Lewis (1986, pp. 192-263) – that the much‐vaunted cases that were given by Kripke (1980) as paradigm cases of metaphysical necessity are not cases of absolute necessity: the sentences in question are true relative to some semantically admissible counterpart relations and false relative to others. Now, of course, we can pick and choose from the Lewis menu, and we might choose to pick from it very little indeed. We might abandon concern with conceptual analysis. We might abandon the idea that conceptual analysis is to be strongly constrained by pre-theoretical opinion. We might (taking a leaf out of the book of contemporary meta-ethics) embark on ‘analytic’ projects in which the analysanda are philosophical conceits – in the present case, that of metaphysical necessity. We might postulate a multiverse such that the broadest modality that it offers is physical modality. And we might even build into our analyses – of possibility, pain, or colour – the version of the physical facts that is given by what we take to be our best, currently available, physical theory. If I have understood correctly, Wilson – quite consciously and happily – departs from Lewis’s modal philosophy, and his approach to analysis in general, in all these ways. However, recall that my point was this: that we might consider whether it is to Lewisian reductive analysis rather than to Quinean elimination that the proponent of QMR ought to look in order to articulate the philosophical significance of the demodalized basis for modality. But now it turns out that, in Wilson’s project, massive chunks of Lewis’s thinking about the analysis of modality are also not to the point. So, if QMR is to be deployed neither in service of the Quinean eliminative agenda nor in service of (a large part of) Lewis’s project of analysing modality, how – within the sphere of reduction-tolerant modal philosophy – is it to be deployed? I think that I know how to state an answer to that question. QMR, as deployed in The Nature of Contingency, gives us a metaphysical analysis or reduction of metaphysical modality. But is there really a well-motivated position of this kind in the space of reduction-tolerant modal philosophy? Certainly, it is widely held – and perhaps pre-eminently so by Williamson (2013) – that metaphysical modalizing is a thoroughly well-motivated, important, and even scientifically indispensable endeavour. Joining Quine, I disagree: but let us suppose, for the moment, that it is so. Next, let us note that Williamson does not seek to reconcile the stated, positive, appraisal of metaphysical modalizing with any kind of reduction: he is, in all presently relevant senses, a primitivist about metaphysical modality. So my question is whether there is, then, a playable fourth position here: one that would praise metaphysical modalizing rather than bury it (contra Quine and more sanguine than Lewis) and that takes no modality as primitive? If I have understood Wilson correctly, he believes that there is such a position and that the metaphysical basis of it is the body of facts described by QMR. But I cannot see it. In light of QMR, the metaphysical naturalist ought to be saying that the foray into metaphysical modalizing has been a kind of mistake, even if an informative one. We thought – for did not our trusty intuitions compel us to think? – that there was in prospect a new and important conception of modality. This distinctive metaphysical modality was not to be the modality that is immediately or obviously the modality manifest in the language of scientific communication. This distinctive modality, discerned by philosophical reflection, was to be other than narrowly logical, other than analytic and other than epistemic: and yet it was to be absolute and – so, crucially – more expansive (in extension) than we had hitherto taken the immanent scientific modalities to be. But if Wilson, or others, have shown that nothing fits the bill – that there is no absolute and objective modality more extensive than those that are manifest in the philosophically innocent language of scientific communication – then I am, at a loss to see what should prevent us from retiring metaphysical modality. As a first step, we should abandon that distinctively philosophical conceit and then accommodate, as we see fit, the modalities as they are presented to us in scientific communication – which is to say, as physical, nomological or dispositional modalities. And, for Wilson, of course, what unifies those is QMR. My main issue with The Nature of Contingency, then, is meta-philosophical. QMR is positioned to occupy a region of modal philosophy which, in my view, is not there to be occupied. If there is a fully satisfactory modal philosophy afoot here, it is one that is eliminative of the notion of metaphysical modality. It even crosses my mind that Wilson will feel that he can easily take that point – caring more about whether QMR ‘works’ than about whether we take its significance to be eliminative or not. My view is that this is not where the philosophical priorities lie: modal philosophy as a whole would be much better off were metaphysical modalizing explicitly put in its place. But my taking that view is, of course, perfectly consistent with my recognizing and acknowledging the virtues of the construction of QMR itself. So, finally, I am keen to make clear that I learned a great deal from this book, and that I found much in it to agree with and more to admire. The Nature of Contingency deserves to be read by researchers in the many fields that it covers and will – I expect – be widely appreciated for the imaginative and impressive work that it is. I am most grateful to Jospeh Melia for his comments on an earlier version of this review. References Divers John , Fletcher Jade 2020 , ‘(Once again) Lewis on the Analysis of Modality’ , in Synthese 197 . Google Scholar OpenURL Placeholder Text WorldCat Everett Hugh 1973 , ‘The Theory of the Universal Wave Function’, in DeWitt B. , Graham N. , The Many-Words Interpretation of Quanum Mechanics ( Princeton NJ : Princeton University Press ).. 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TI - The Nature of Contingency: Quantum Physics as Modal Realism, by Alastair Wilson
JF - Mind
DO - 10.1093/mind/fzab004
DA - 2021-03-19
UR - https://www.deepdyve.com/lp/oxford-university-press/the-nature-of-contingency-quantum-physics-as-modal-realism-by-alastair-kTrZ0H6izI
SP - 1357
EP - 1364
VL - 131
IS - 524
DP - DeepDyve
ER -