In The Nature of Contingency: Quantum Physics as Modal Realism, Alastair Wilson introduces and defends quantum modal realism. This is the conjunction of two views: Everettian quantum mechanics and a modal realist interpretation of this first view, inspired by the work of David Lewis. Everettian quantum mechanics, on the one hand, is a theory of quantum mechanics. It entails that the universe around us is but one of a greater reality encompassing a gigantic number of universes usually referred to as ‘worlds’ or ‘branches’, which co-exist and structure this broader reality: the so-called quantum multiverse. Lewisian modal realism, on the other hand, is a philosophical view about the ontology of reality and modality. It states that the actual world is just one among all the equally real possible worlds. What we refer to as the ‘actual’ world is simply our world, just as agents inhabiting other possible worlds view their own world as ‘the’ actual world. This prima facie odd approach was motivated in various ways by Lewis. Perhaps, most importantly, the view aims at explaining the remarkable efficiency of the possible worlds semantics at describing modal facts and addressing issues with ersatzist interpretations that replace worlds with abstract entities of some sort.
The two views—Everettian quantum mechanics and modal realism—involve a similar radical move: they ask us to recognize that the domain of existence is much larger than the one universe we usually identify with reality. True, the domain of existence posited by modal realism considerably exceeds the one posited by Everettian quantum mechanics. However, the two domains are so vast that this difference does not matter when it comes to getting to grip with their sheer size. The most important difference between the two views is probably the nature of the new worlds posited. According to modal realism, what comes in greater quantity is the number of possible worlds. According to Everettian quantum mechanics, as usually interpreted, the actual world contains far more concrete worlds than the single concrete world we observe around us.
Wilson challenges this standard interpretation of Everettian quantum mechanics and builds a strong case for an alternative approach. The book does not offer any defence of Everettian quantum mechanics; rather, Wilson takes the view for granted in order to develop a novel philosophical interpretation of the theory. The central idea guiding him is that Everettian quantum mechanics can be morphed into a modal theory very much like the modal realism of Lewis. According to the quantum modal realism he advocates, the branches of Everettian quantum mechanics are identical to the possible worlds of (an amended version of) modal realism. This implies that quantum mechanics is not only a theory about what there is, but is also about the structure and extension of modal space. Wilson convinced me that this view offers a powerful explanation of both modality and quantum mechanics, and is definitely one worth considering.
The book is divided into six chapters as follows: After an introduction presenting the overall architecture of quantum modal realism, Chapter 1 echoes Lewis’s On the Plurality of Worlds () by discussing Lewis’s modal system and how to ‘quantize’ it. Chapter 2 focuses on the causal structure of the multiverse and whether the branches should be analysed as overlapping or diverging. Chapter 3 investigates the notion of chance and how to construe objective chance in light of quantum modal realism. Chapter 4 looks into the ontology of laws of nature, arguing that different sorts of laws can have different modal status. Some laws apply to the fundamental quantum state, which is regarded as necessary, and others to all the branches of the derivative quantum multiverse, which make the multiverse necessary too; other laws are found in subsets of branches only and are thus contingent. Chapter 5 discusses the reconciliation of quantum modal realism with the standard interpretation of Everettian quantum mechanics according to which the number of worlds composing the multiverse is not determined. Finally, Chapter 6 examines some consequences of this view for cosmological fine-tuning arguments and anthropic reasoning.
The book offers a rich analyse of these topics, to which I cannot do justice in the present review. In what follows, I will restrict myself to discussions of the nature of modality, the central topic analysed in the book; but I could have picked many other intriguing issues, such as the role of indeterminacy in the multiverse, or the grounding relation used to relate the fundamental universal quantum state to the derivative multiverse.
Wilson parts company with the view, popular in metaphysics, that the range of metaphysical possibilities exceeds the range of natural and linguistic possibilities. Wilson must deny, for instance, that the multiverse could have failed to exist. He is not the first to deviate from the ‘metaphysical modality’ orthodoxy—the view that metaphysics modality does not reduce to another sort of modality. He thereby falls within a category of thinkers who challenge, at least to some degree, this modal orthodoxy. One may think, for instance, of Alan Sidelle (), who points out that philosophers have been too quick in inferring from the work of Saul Kripke () the existence of the new sui generis category of metaphysical modality. Sidelle deflates the category of metaphysical modality, arguing that it merely refers to a particular sort of linguistic modality that has, really, nothing to do with the extra-linguistic nature of the world. Likewise, Wilson restricts the domain of metaphysical possibilities to the domain of natural possibilities. Thus, both Sidelle and Wilson deny the existence of an autonomous category of metaphysical modality irreducible to another category of modality.
However, Wilson reassures metaphysicians that quantum-mechanical possible worlds can still be identified with metaphysically possible worlds. In my view, it would have been interesting to explore an alternative view, namely, to go against the orthodoxy in contemporary metaphysics and bid adieu to metaphysical modality. Indeed, the empirical attitude toward modality advocated in the book lays the groundwork for a full-fledged naturalistic view of modality that uses empirical sciences, rather than modal logic and modal intuitions, as a guide to the range and nature of possibility.
To be fair, Wilson explicitly states that he remains neutral regarding the reductionist and eliminativist approaches to metaphysical modality, and that the reader should feel free to read the book with their favourite view in mind (p. 24). But, in my opinion, it is one beautiful feature of the proposal that it allows for an analysis of modality that no longer requires metaphysically possible worlds. Indeed, as objective modality is fully, and naturalistically, captured by one of our best theories in physics according to quantum modal realism, one might wonder: why should we continue to bother with the category of metaphysical modality and the formal properties of modal logic systems elaborated to account for modal intuitions?
This relates to the issue of advanced modalizing (pp. 34–40). The expression originally referred to a class of cases of intuitive possibilities that may not be identified with possible worlds. They arise from our intuitions about the modal status of entities located in other possible worlds. For instance, assuming that modal realism is the correct view of reality, it seems that we nonetheless have the intuition that we could have been living in a universe made of a single concrete world—and thus that the entities located in other worlds could have failed to exist. However, this possibility is not easily described in modal realism, which, at first sight, identifies possibilities to possible worlds, seemingly forcing us to either dismiss apparent possibilities associated with alternative structures of the modal space or deny that possibilities correspond with possible worlds. To deal with the issue, Wilson dives into the classical literature in the philosophy of modality. He discusses, for instance, Philip Bricker’s () suggestion to recognize arbitrary mereological sums of primitive worlds to be worlds themselves, as well as the idea of using counter-pairing relations, namely, functions from the individuals in the multiverse to individuals in the multiverse. Wilson expresses his preference for the latter approach but emphasizes that the very same solutions are available for both modal realists and quantum modal realists.
This is one way to proceed. But quantum modal realism, unlike Lewisian modal realism, is not a purely a priori approach. What if all the cases of possibilities (apparently) at odds with Everettian quantum mechanics, including cases of advanced modalizing, were not genuine possibilities after all? Do we really need to satisfy our intuitions about what is possible, together with the modal logic systems developed to account for those intuitions? It is certainly standard among philosophers of modality to aim at modelling the ontology of modality on the epistemology and formal logic of modality—say, by accepting the linkage from (mind-dependent) ideal conceivability to (mind-independent) genuine possibility or the relevance of the popular S5 modal logic. The usual motivation behind this standard approach is that this is the best we can do; the only alternative would be to be sceptical about modal knowledge.
But with quantum modal realism, it is no longer true that this is the best that we can do: an empirically established theory now competes, forcefully, with a priori considerations to legitimately describe the modal structure of our universe. The situation is remarkably similar to that of our conception of time during the twentieth century: with relativistic physics, we learned that we need to abandon certain intuitions about the universality of time. It is now admitted that demanding our ontology of time to mirror our a priori, non-relativistic epistemology of time is a mistake. I believe the same to be true with modality in the context of quantum modal realism: if quantum mechanics is genuinely a modal theory, then we lose the original motivation for using modal intuitions and modal logic as primary guides to probe the nature of modality.
This leads us into another discussion in the book that I found a bit perplexing: whether it is possibility or necessity that is prior, ontologically speaking (Section 0.3). Wilson believes that, at least at first sight, there are reasons to take possibility to be prior to necessity. And he then argues that the tables have turned with modal realism and quantum modal realism. In quantum modal realism, the contingent worlds constituting the multiverse are less fundamental than the necessary, absolutely fundamental quantum state, and so necessity turns out to be more fundamental than possibility. But why believe that one notion is more fundamental than the other in the first place? And do we really need to accept, with Wilson, that the fundamental quantum state is necessary?
A more appealing approach, to my mind, is to defend the claim that the universal quantum state has no modal status at all and, hence, is neither necessary nor contingent; necessity and contingency are on an equal footing. Both contingency and necessity emerge at the derivative level from a non-modal ontology: contingency as the variation across quantum branches; necessity as the existence in many or all branches—depending on the kind of necessity under consideration. In claiming that both contingency and necessity are derivative phenomena, we can avoid imbuing one notion with priority over the other. Note that I am not claiming that it is bad to ascribe necessity to the fundamental quantum state. But I want to emphasize that there is no obvious reason to do so if quantum modal realism is the correct picture of reality. After all, if individuals, worlds, and contingency all emerge from the universal quantum state, why not necessity?
In summary, I think Wilson could have taken a step further in the development of an entirely naturalistic theory of modality based on quantum modal realism. Moreover, I do think there are reasons in this approach to uproot the necessity as well as the contingency of the fundamental quantum state, relocating both to the non-fundamental level of description, namely, in the emerging multiverse. Nevertheless, I find the general proposal and the vision of reality that underlies it staggering. The book is impressive in its scope, highly innovative, and a must-read for anyone interested in either the ontology of quantum mechanics or the philosophy of modality. Those familiar with both fields of research will certainly take great pleasure in exploring this novel and beautiful philosophical system.
Baptiste Le Bihan
University of Geneva
Bricker, P. : ‘Island Universes and the Analysis of Modality’, in G. Preyer and F. Siebelt (eds), Reality and Humean Supervenience: Essays on the Philosophy of David Lewis, Lanham, MD: Rowman and Littlefield, pp. 22–54.
Kripke, S. : Naming and Necessity, Cambridge, MA: Harvard University Press.
Lewis, D. : On the Plurality of Worlds, Oxford: Blackwell.
Sidelle, A. : Necessity, Essence, and Individuation: A Defense of Conventionalism, Ithaca, NY: Cornell University Press.