Quantum mechanics has always presented a problem for scientific realists. For it does not seem to offer a coherent picture of the world. I think it does, and that there is a viable quantum mechanical model of reality. But it cannot be constructed, given the continuity constraints that we usually place on realistic models. To be a quantum mechanical realist, one has to be willing to accept that there are two fundamentally different kinds of processes occurring in nature: continuous, but quantum mechanically indeterminate, energy transfer processes and discontinuous, but spatially extended, changes of state. The unwillingness of philosophers to accept that there is such a distinction has caused many of them to doubt the reality of Schrödinger waves, and the processes of their generation and collapse. I am fully aware that I am treading on well-trodden territory, and that the technical difficulties that must be faced by anyone who is not a specialist in the field are immense. Nevertheless, I think there is a fairly clear metaphysical position on these issues derivable from the kind of essentialist realism outlined in the previous chapter, and I ask readers to bear with me. First, realism about Schrödinger waves requires the belief that particles are always transmitted as quantum mechanically indeterminate waves, but always act as classically determinate particles on whatever absorbs them. The probability of absorption by a given absorbing object (e.g. measuring instrument) at a given location is determined by the value of the Ψ-function at this point.
Quantum mechanics has always presented a problem for scientific realists. For it does not seem to offer a coherent picture of the world. I think it does, and that there is a viable quantum mechanical model of reality. But it cannot be constructed, given the continuity constraints that we usually place on realistic models. To be a quantum mechanical realist, one has to be willing to accept that there are two fundamentally different kinds of processes occurring in nature: continuous, but quantum mechanically indeterminate, energy transfer processes and discontinuous, but spatially extended, changes of state. The unwillingness of philosophers to accept that there is such a distinction has caused many of them to doubt the reality of Schrödinger waves, and the processes of their generation and collapse. I am fully aware that I am treading on well-trodden territory, and that the technical difficulties that must be faced by anyone who is not a specialist in the field are immense. Nevertheless, I think there is a fairly clear metaphysical position on these issues derivable from the kind of essentialist realism outlined in the previous chapter, and I ask readers to bear with me. First, realism about Schrödinger waves requires the belief that particles are always transmitted as quantum mechanically indeterminate waves, but always act as classically determinate particles on whatever absorbs them. The probability of absorption by a given absorbing object (e.g. measuring instrument) at a given location is determined by the value of the Ψ-function at this point.
Quantum mechanics has always presented a problem for scientific realists.For it does not seem to offer a coherent picture of the world.I think it does, and that there is a viable quantum mechanical model of reality.But it cannot be constructed, given the continuity constraints that we usually place on realistic models.To be a quantum mechanical realist, one has to be willing to accept that there are two fundamentally different kinds of processes occurring in nature: continuous, but quantum mechanically indeterminate, energy transfer processes and discontinuous, but spatially extended, changes of state.The unwillingness of philosophers to accept that there is such a distinction has caused many of them to doubt the reality of Schrodinger waves, and the processes of their generation and collapse.I am fully aware that I am treading on well-trodden territory, and that the technical difficulties that must be faced by anyone who is not a specialist in the field are immense.Nevertheless, I think there is a fairly clear metaphysical position on these issues derivable from the kind of essentialist realism outlined in the previous chapter, and I ask readers to bear with me.First, realism about Schrodinger waves requires the belief that particles are always transmitted as quantum mechanically indeterminate 1.
This chapter develops metaphysics to include quantum mechanical realism. Quantum mechanics has always been the main obstacle to acceptance of scientific realism, and hence to any metaphysics based on scientific realism. Realism about Schrdinger waves requires the belief that particles are always transmitted as quantum mechanically indeterminate waves, but always act as classically determinate particles on whatever absorbs them. Acceptance of a realistic theory of Schrdinger waves, and of the very rapid or instantaneous changes of state that quantum theory implies, commits one to the view that all transitions from indeterminate to determinate states are temporally irreversible, apparently in contradiction to the widely accepted T-symmetry thesis. Realism about Schrdinger waves already implies a kind of non-locality, because what was spatially distributed before the event of determination suddenly becomes localized in that event. Quantum realism implies that there is a common species of elementary physical causation that is probabilistic, and directed from one determinate state of affairs to another.
Introduction It is not exaggeration to claim that one of the major divides in the foundations of non-relativistic quantum mechanics derives from the way physicists and philosophers understand the status of the wave function. On the instrumentalist side of the camp, the wave function is regarded as a mere instrument to calculate probabilities that have been established by previous measurement outcomes. On the other “realistic” camp, the wave function is regarded as a new physical entity or a physical field of some sort. While both sides agree about the existence of quantum “particles” (the so-called theoretical entities), and therefore reject the radical agnosticism about them preached by van Fraassen (1980), the various “realistic” (and consequently, instrumentalist) philosophies of quantum mechanics are typically formulated in different, logically independent ways, so their implications need to be further investigated. For instance, on the one hand it seems plausible to claim that a realistic stance about the wave function is not the only way to defend “realism” about quantum theory. One can support a “flash” or a “density-of-stuff ” ontology (two variants of GRW), or an ontology of particles with well-defined positions (as in Bohmian mechanics), as primitive ontologies for observer-independent formulations of quantum mechanics (Allori, Goldstein, Tumulka and Zanghí, 2008). “Primitive ontologies”, as here are understood, are not only a fundamental ground for other ontological posits, but also entail a commitment to something concretely existing in spacetime (see also Allori, 2013). On the other hand, however, it is still debated whether such primitive ontologies can be autonomous from some form of realism about the wave function (Albert, 1996). In order to discuss this problem, we begin with a preliminary clarification of the meaning of “realism” and “instrumentalism” in physics, which are often subject to ideological and abstract discussions that often have little to do with the practice of physics (Section 9.2).
According to scientific realism, science gives us an approximately true description of what the world is like. But what does it mean? In this chapter, we focus on the ontological and metaphysical aspects of this discussion. That is, we are concerned with the following questions: what there is, according to our best scientific theories? And how are these things that theories say exist? We start from the assumption that a genuine scientific realism must deal with these issues. Taking non-relativistic quantum mechanics as a case study, we discuss some of the challenges currently faced by a genuinely realistic stance. We argue, first, that in the ontological aspect, realists are in the same boat as science, that is, without sufficient epistemic justification to adopt the belief in the entities postulated by a single quantum theory, given that experience (currently) is not able to decide between rival theories; in the metaphysical aspect, realists also encounter problems with theory choice, given the possibility of associating more than one metaphysical theory with the description of the entities postulated by each quantum theory. The same goes for structuralist alternatives, since, even if we accept that scientific theories are committed to the existence of structures, but not objects, we still do not know what structures are, metaphysically. Finally, we move forward in the discussion with the meta-Popperian method, which serves to reduce the metaphysical alternatives associated with the ontological postulates of scientific theories. We may not know what the correct alternative is, but we have been able to progressively check which metaphysical alternatives are incompatible with each scientific theory.
This book is a treat, a collection of essays about the relationship between consciousness and quantum mechanics that is guaranteed to move the needle: serious, original, and relatively accessible. The essays develop cutting-edge views by thought-leading authors on questions that matter for the philosophy of physics and the philosophy of mind, and while to follow along you need to know what supervenience is and what an eigenstate is, the majority of essays should be accessible to advanced undergraduates (i.e., those who have taken a rigorous introductory course in quantum mechanics as well as a course or two in the philosophy of mind).Why care about the relationship between consciousness and quantum mechanics? If you are coming from physics, your point of entry is probably the measurement problem. When you let quantum systems evolve according to the Schrödinger equation, they enter into superpositions of mutually incompatible states. How can this be reconciled with the testimony of our senses that such systems are always in one state or the other when we measure them (with a probability distribution given by the Born rule)? This is the measurement problem, and it concerns consciousness because it draws from the testimony of our senses.If you are coming from the philosophy of mind, your primary concern is probably the mind-body problem: the task of accounting for how consciousness arises out of or fits into physical reality. If consciousness is reducible, how might the details of quantum mechanics make it harder, or easier, to establish that reduction? If consciousness is irreducible, how might the details of quantum mechanics make it harder, or easier, to explain why consciousness exists, or to find some work for it to do in the world? You may also be concerned with the structure and function of consciousness: for example, you may hold that minds can do things that classical digital computers cannot, and take it that we must look to the quantum realm to discover how.If you are not already entrenched, you might just wonder what kind of world we live in. The mechanistic philosophy of centuries past seemed to disenchant the world, rendering it a place of barren corpuscles banging into one another, leaving no room for mind or meaning, except perhaps as patterns in the collisions, or useless epiphenomena. But quantum mechanics and its relativistic generalizations force a rethinking of the mechanistic picture from the ground up. Once the dust has settled—if it ever does—we may find reasons to relegate consciousness once again to the shadows. But we may also find reasons to give it pride of place. If you are after scientific reasons to reenchant the world, this is a good place to look.The entries in this volume address these questions and others besides. The volume is organized into three parts. The first part contains four essays about the idea that consciousness causes (or constrains) quantum collapse. The second part contains eight essays discussing other ways that consciousness might contribute to theories of quantum mechanics. The third part contains five essays about how quantum mechanics might contribute to an ultimate theory of consciousness. I’ll follow the book’s order.In chapter 1, David Chalmers and Kelvin McQueen develop a new consciousness-triggered collapse interpretation of quantum mechanics. On their approach, consciousness has a strict physical correlate (perhaps given by quantum integrated information theory), it is approximately superposition-resistant, and collapse dynamics are continuous (à la Philip Pearle). They respond to the standard objections that a consciousness-collapse link makes the collapse mechanism imprecise and requires dualism, as well as to worries about Quantum Zeno, among others. Crucially, their approach allows that superpositions of consciousness are possible: their motivation is to give the view a run for its money and to find work for consciousness to do in the universe, not to rid the world of all traces of superposed consciousness. One question: In taking consciousness itself to be superposition-resistant (rather than the environmental systems it asks questions about, as Henry Stapp suggests), we appear to favor internalism over externalism. What would an externalist variant look like? Another question: How exactly would experimental results—for example, that simple enough systems with maximal Φ remain in coherent superposition—constrain our theoretical options? If to accommodate this we have to impose an experimentally determined threshold for the minimal absolute Φ for consciousness, will that mean that on the physicalist version of the view, the property we privilege for superposition-resistance is just an arbitrary collection of atoms?In chapter 2, Elias Okon and Miguel Angel Sebastian defend a consciousness-collapse theory that they call the Subjective-Objective collapse model. Like Chalmers and McQueen, Okon and Sebastian invoke a Pearle-style continuous collapse dynamics and think of consciousness as only weakly superposition-resistant. A key contribution is the treatment of a worry about multiple realizability (i.e., the way that some property of biological neurons and some other property of artificial neurons might both realize the state of being in pain). The problem is that it is not straightforward to define Hermitian operators whose eigenstates are disjunctions of the eigenstates of other Hermitian operators: you must ensure orthogonality (“being red or being a circle” is not orthogonal to “being red or being square”). In answer, the authors point out that we don’t need each multiply realizable phenomenal state to be an eigenstate of our collapse operator; it suffices that each way of realizing each phenomenal state is an eigenstate. One question is whether this problematizes the dancing qualia argument. A second worry is that we still must find a suitable basis for Hilbert space. But theories of consciousness may constrain both position and momentum: for example, both distances between neurons and how energy is transferred. We can talk about wave packets, but these will not get us eigenstates of a Hermitian operator.In chapter 3, J. Acacio de Barros and Carlos Montemayor suggest that if a mentality-triggered collapse theory is what you want, you should opt for intentionality rather than phenomenality. This is because, first, we have evidence from psychophysics that there is unconscious perception. Second, the first-personal data that motivate collapse theory concern intentionality: we remember only recording definite values; our decisions about what to measure affect the outcomes we see. First question: Can the reasons we have for thinking that there is unconscious intentionality extend into a case for unconscious primitive intentionality? Another question is how the proposal here would differ from Stapp’s treatment of what questions a mind asks of the universe. A third is how we contend with Quantum Zeno, the effect that forces both Chalmers-McQueen and Okon-Angel Sebastian to say that consciousness is only weakly superposition-resistant. Those authors note the possibility that the relevant superposed states of consciousness might overflow access consciousness (in Ned Block’s sense). We may have an easier time ensuring that access conscious states only rarely enter into superposition, if we set them back from our front line of superposition-resistant states.In chapter 4, Adrian Kent explores objective collapse theories—mass-dependent continuous spontaneous localization models. Consciousness may not be in the spotlight on these theories, but they should at least ensure that collapse happens faster than the time it takes for a stimulus (e.g., a system of several photons) to make it from a retinal impact to consciousness. Angelo Bassi, Dirk-André Deckert, and Luca Ferialdi (2010) argue that for a range of theories this constraint is respected, with collapse for a system of several photos happening at around 100 ms. Kent problematizes their argument, showing that their estimation may be off by an order of magnitude or more. Kent’s point is compelling: more work is needed to refine the estimate and to conceptually clarify the import of the constraints from perception.Part 2 consists of eight essays on various ways that consciousness might figure in or constrain quantum theories. In chapter 5, Phillip Goff argues that 3N-dimensional wave function monism cannot accommodate ground truths about consciousness, pressing on worries that the view imposes too few structural constraints to allow for the unambiguous recovery of ordinary 3D reality. While I agree that wave function monistic accounts of ordinary reality (e.g., Albert’s functionalism or Ney’s priority monism) yield an explanatory gap (even given the 3D-ish structure of GRW jumps), I was puzzled here. All type-B physicalists confront an explanatory gap where consciousness is concerned. Isn’t this a point in support of wave function monism, since it amounts to a companions-in-guilt argument against opposition from other type-B materialists?In chapter 6, Peter Lewis raises some sensible worries about the use of phenomenal consciousness as an unanalyzed primitive or special case in quantum theory. He objects to the suggestion made by some Bohmians that we can be directly aware of the particles in our brains. Another target is John von Neumann, who appeals to intuitions about psychophysical connections to justify a collapse hypothesis. These are nice points: I would be curious to know whether Lewis thinks they apply to the justifications for mentalistic collapse theories from part 1.In chapter 7, Jenann Ismael first argues that the hard problem of consciousness is of no concern to physics per se: “If consciousness enters the problem space of physics, it does so by making a difference to the behavior of physical objects.” She then offers some clarifying remarks about how we might reconcile Van Gelder-style dynamicist conceptions of mentality with (virtual) machine functionalism. I find much to agree with here, though I do wonder how strictly to take Ismael’s claims about the irrelevance of the hard problem. If there is a tie between two theories of physics over predictive accuracy, elegance, and naturalness, but one yields a more satisfying analysis of consciousness than the other, are we not even allowed to treat that as a tie-breaker?In chapter 8, Shan Gao defends a mentalistic characterization of the measurement problem in reply to Andrea Oldofredi (2019). The question is whether the key premise of the measurement problem should be that measurements take on definite values or only that we (definitely) experience them as taking on definite values. Of course, our reasons for thinking that they do arise from our seeing or remembering that they do, but we can distinguish between the claim and our evidence for it. Gao argues that the underlying mentalistic claims are more inextricable from the dilemma than that. Many dilemmas involve premises for which the evidence is perceptual, though they make no mention of perception. Here, however, solutions to the problem require special attention to the nature of that evidence.In chapter 9, Paul Skokowski illustrates one reason that we must pay special attention to the nature of that evidence. Jeffrey A. Barrett’s (1999) bare theory is a minimalistic reading of Everett on which measurement outcomes are only definite if they obtain on all branches. One question is whether this means that observers have blatantly false disjunctive beliefs: let someone measure an electron’s spin and then ask, Do you believe that you got a definite measurement outcome, either “UP” or “DOWN”? The observer will say yes, because on each branch they saw one and not the other. However, by the lights of the theory, this disjunctive belief is false. Skokowski’s insight is that while the observer definitely says yes, it does not follow (by the lights of the bare theory) that the observer has a (definite) false belief, since the neural realizer for the disjunctive belief will be different on each branch and therefore not correspond to a common eigenstate (compare the worry from chapter 2 about multiple realizability). But isn’t this just to move from the frying pan to the fire, as it seems now that our observer has no (definite) beliefs at all?In chapter 10, Michael Silberstein and W. M. Stuckey advocate for a psi-epistemicist perspective that vindicates many phenomenological intuitions, including the one that experimental outcomes are definite (and intersubjectively consistent). The approach requires that we rethink naïve realism about the wave function and that we rethink mind-body duality, in favor of a neutral monist ontology governed by constraints. This is an exciting approach, though I’m not fully clear on how to differentiate neutral monism from its idealist, dual aspect, or panpsychist cousins.In chapter 11, Michael Bitbol suggests a phenomenology-first approach to interpreting quantum mechanics. Bitbol favors a QBist participatory realism, tempered by phenomenology, that could perhaps also be described as neutral monist. He notes that the phenomenology of embodiment (à la Merleau-Ponty) acquaints us with how something can be both active and passive, both minded and incarnate, and he traces the error of believing in a separate, physical world (an error participatory realism corrects) to the tendency of self-separation: the tendency to try to project ourselves beyond ourselves, discussed in Heidegger, Sartre, and Michel Henry. These phenomenological insights are enlightening, though I am still not quite clear on how the underlying (neutral?) ontology works. Who, precisely, are the observers? Do electrons have feelings, or are they just fictions we use to cope with ours?In chapter 12, Lucien Hardy describes an experiment in which humans might be used to determine the settings in a Bell experiment. Hardy suggests that the outcome may be a violation of quantum theory consistent with the Bell inequality. He argues that this result would support (superdeterministic) mind-body dualism and also could serve as a new more objective Turing test. This is a fascinating proposal: it remains only to do the experiment.This brings us to part 3: accounts that move from quantum mechanics to consciousness, rather than conversely. In chapters 13 and 14, Roger Penrose and Stuart Hameroff give a comprehensive account of the current state of their ORCH-OR theory of quantum collapse and consciousness. Among other things we find here a nice presentation of Goodstein’s theorem as an example of a theorem that is noncomputable but easy enough for humans to understand, and a nice discussion of how retroactivity might equip us with something to say to Benjamin Libet about the role of consciousness in action selection. I’ll note, though, that I asked ChatGPT to explain Goodstein to me and it did a fairly good job, suggesting something like understanding. On Libet, I’m not sure what exactly we need retrocausality for: Why not just content ourselves that awareness of the action plan leaves time for a veto, as Libet and others have suggested?In chapter 15, Basil J. Hiley and Paavo Pylkkänen explore another route to a comprehensive theory of consciousness, drawing from the materials of a theory of quantum reality. Here, that theory is the Bohmian implicate order picture, with special attention to the notion of active information that arises there. This is another approach with neutral monist overtones, but here the wave function of a system behaves something like a customized hylomorphic form for the system while at the same time imbuing it with intentionality. There are interesting claims here about the continuity between life and mind, the unity of consciousness, and protopanpsychism, as well as some speculation of how the theory might align with the various scientific theories of consciousness inspired by psychophysics, like higher-order or global workspace theories.In chapter 16, William Seager offers a helpful discussion of the kinds of intelligibility at issue in these debates, distinguishing between mundane, mathematical, and metaphysical intelligibility, weaving together insights from Leibniz and others to argue that a Bohmian approach offers the best bet for a metaphysically intelligible world view. I sympathize with the positive aspects of his argument, though I think that other papers in this volume showcase the metaphysical intelligibility of rival approaches.Finally, in chapter 17, Lee Smolin describes a completion of quantum mechanics, also echoing Leibniz, which could support a theory of quantum gravity and which offers a new conception of consciousness: the causal theory of views, alongside the principle of precedence. This yields a novel sort of protopanpsychism in which the world is a collection of (background-independent) causal views, where views are conscious when they have a few extra features, including being unprecedented and being maximal. This is an exciting approach. One question is how we reconcile the claim that experiential states are maximal with the (mental) problem of the many, the problem that there are ties between composites (that differ over a stray ion or two) over which has best claim to be me.In closing, this is a fantastic volume. I commend Shan Gao for his editorial work: he would make a wonderful DJ. There are subtle symmetries in the way that each entry segues to the next, and the collection has a remarkable coherence.
David Lewis famously quipped that he was willing to take metaphysical lessons from quantum mechanics only when it cleaned up its own act by providing interpretations or modifications of the formalism shorn of appeal to consciousness, irreducibly macroscopic notions, or outright instrumentalism. By the first decade of the twenty-first century, multiple approaches to quantum mechanics had been developed to a degree which met Lewis’s challenge: dynamical-collapse theories, Bohm-style hidden-variable theories, and Everett-style many-worlds theories, for all their respective problems and challenges, all offer ways of understanding quantum mechanics that metaphysicians can reasonably engage with. And so the last twenty years have seen a resurgence in the metaphysics of quantum mechanics, the question of what ontology and ideology is appropriate to a quantum universe. The metaphysics of quantum mechanics is not independent of the quantum measurement problem but does not coincide with it, either: there is a widespread view that adopting, say, the Everett interpretation still leaves significant ontological questions undecided, and conversely, a given answer to those questions might answer them for a dynamical-collapse theory too.Wave function realism has become a leading contender for the metaphysics of quantum mechanics, particularly in the case of dynamical-collapse and Everettian approaches. The starting point is nonrelativistic quantum mechanics expressed in configuration space: mathematically, the subject matter of the theory is a complex function of 3N variables, evolving under the Schrödinger equation (perhaps interrupted by occasional stochastic collapses). Traditionally those 3N variables are interpreted as the three coordinates of position of N particles—but in wave function realism, they are interpreted as points in a new, high-dimensional space. Quantum mechanics, according to the wave function realist, fundamentally concerns the undulations of the wave function in this space; three-dimensional goings-on are emergent, high-level, derivative.Alyssa Ney has been a leading figure in the development of wave function realism for most of its existence. Her articles advocating and developing the position have been highly influential, and the present book can be seen as the culmination of this line of her research and is the definitive statement of wave function realism as of 2021. Despite its comparative brevity, the book is too rich to be adequately described—far less critiqued—in this brief review; my goal is to give an overview of the main themes and arguments, and to make a few critical remarks along the way.The World in the Wavefunction is not formally divided into parts, but it can profitably be thought of that way. Chapters 1 through 3 develop the core case for wave function realism. Chapter 4 defends the position from the frequent criticism that it is restricted to nonrelativistic physics, by providing a version of wave function realism appropriate to quantum field theory. And chapters 5 through 7 are an extended exploration of how the observed three-dimensional world can be understood as emerging from a high-dimensional fundamental reality.The case Ney makes in chapters 1 through 3 for wavefunction realism is modest and cautious. In chapter 1 she provides a fairly standard—though elegantly expressed—case for the prima facie plausibility of wave function realism simply based on the fact that standard formulations of quantum mechanics are written in 3N dimensions, and on the (related) fact that entanglement means we cannot interpret quantum mechanics simply in terms of spatially localized particles and nothing else. But in chapter 2 she explicitly argues against any attempt to strengthen these plausibility arguments into reasons why wave function realism must be adopted; indeed, she provides a detailed and sympathetic taxonomy of the alternatives to wave function realism that have been developed, all of which she regards as entirely viable. (Chapter 2 of the book could profitably be used as introductory reading for a general graduate course on quantum metaphysics.) For Ney, the question of which metaphysics to adopt is a standard metaphysician’s question, to be answered by weighing strengths and weaknesses of the various alternatives against one another.In chapter 3, Ney identifies two such strengths for wave function realism. First, it is separable: we can specify the complete state of the universe point-by-point on the fundamental space, without any need to appeal to irreducible holism. Second, it is dynamically local: expressed in 3N-dimensional space, the Schrödinger dynamics involve no action at a distance.Ney makes the case for separability at length. Her case for dynamical locality is brief: she regards it as a straightforward consequence of the fact that the Schrödinger equation is a differential equation, so that the rate of change of the wave function at a point x depends only on the wave function arbitrarily close to x. But I think there is more worth investigating here: after all, the nonrelativistic Schrödinger equation may be a differential equation, but it is a parabolic differential equation, with no maximum speed of propagation: the rate of change of the wave function at x may depend only on a neighborhood of x, but the value of the wavefunction at x an arbitrarily short time later depends on the wave function everywhere in space. (And this is not—or not straightforwardly—just an artifact of ignoring relativity.)But in any case, why think that a separable, local theory is preferable to the alternative? Ney claims that separability and locality are more intuitive and lead to a more comprehensible ontology—and she makes an unabashed defense of intuitiveness and explicability as theoretical virtues. She is quite explicit that her reasons are pragmatic: intuitive theories benefit students and teachers; they are better suited to help us develop new theories; they speak better to the reasons why many students (Ney included) want to study physics in the first place. The implied metaphilosophy here is fascinating and it would be good to see it developed further in subsequent work: Ney seems to reject the mainstream modern approach to ontology (in which these pragmatic virtues are not obviously relevant to the truth of wave function realism) in favor of something neo-Carnapian where (so far as I can see) the goal of a quantum ontology is to help us get a deeper understanding of quantum mechanics, and that’s all there is to it. This also fits naturally with the pluralism she urges, in which approaches other than hers can also be valuable.Chapter 4—on relativistic generalizations of wavefunction realism—attempts both a conceptual and a technical task. The conceptual task is to argue that wavefunction realism per se is not wedded to any particular theory or choice of Hilbert-space basis within the theory: rather, wave function realism is any strategy that reifies a high-dimensional space in order to produce a separable local quantum ontology. If so, there is no block in principle to a relativistic generalization of the approach: we just need to find some appropriate high-dimensional space to describe the quantum state in relativistic quantum theory.The general philosophical point is well taken and convincingly defended (though it’s important to note that while arbitrary choices of Hilbert-space basis will give a separable theory, there is no reason to assume they will give a dynamically local theory: even something as simple as shifting from the position to the momentum basis in nonrelativistic quantum mechanics transforms the Schrödinger equation into something explicitly nonlocal). Unfortunately, I don’t think Ney succeeds in giving a clear statement of the high-dimensional space she has in mind for quantum field theory. This is one place where her laudable goal of making the book intelligible to nonspecialists becomes problematic: she makes many suggestive and interesting comments on quantum field theory, but there is no explicit, precise, mathematical statement of her view that can be contrasted with the explicit, precise, mathematical representation of the state in nonrelativistic quantum mechanics as a complex function on 3N-dimensional space.In the last part of the book—concerned with the emergence of ordinary three-dimensional ontology—Ney begins (chapter 5) by rebutting influential arguments by Tim Maudlin, Valia Allori, Sheldon Goldstein, and others that it is empirically incoherent for a physical theory not to make precise, nonemergent, nonderivative statements about matter in three-dimensional space (for a theory not to have a primitive ontology, to use the standard term for it). Ney argues—persuasively, to me—that it is perfectly compatible with the way science usually works for a theory to make statements about the observable only in an indirect, derivative way, so that while it might be a desideratum for a theory to have a primitive ontology, it is not compulsory for it to do so in order to be empirically adequate. She goes on, though, to note that a satisfactory quantum metaphysics had better recover the existence of ordinary three-dimensional objects, on Moorean grounds: we are really confident that those objects exist!Chapter 6 is largely critical: an extended exegesis of the functionalist strategy for recovering higher-order ontology developed by David Albert, myself, and others. She raises two main criticisms of the functionalist strategy: it fails to distinguish between the wave function simulating three-dimensional objects and constituting those objects; it is restricted to recovering classical three-dimensional ontology, whereas we want to recover quantum-mechanical phenomena in three dimensions too. (The former objection is a development of criticisms by Bradley Monton, Tim Maudlin, and others; the latter is novel so far as I know.) For reasons of space, I won’t attempt to reply on behalf of the functionalist, but Ney certainly puts the challenge with admirable clarity.The final chapter of the book presents Ney’s positive account of emergent ontology, based not on functionalism but on considerations of symmetry (though she concedes that the separation is not completely sharp). Simplifying Ney’s subtle position: the permutation symmetries of quantum mechanics give a clear dynamical reason to regard certain decompositions of 3N-dimensional space (intuitively: those that decompose it into the N-fold product of Euclidean space) as preferred; that then licenses the metaphysical hypothesis that a localized wave function in 3N-dimensional space has, as its parts, particles in three-dimensional space. If the wave function is not localized (as will be the case generically, at least for the Everett interpretation), then we can think of those particles as being partially localized in various different regions of space.I have to say that I am not convinced this works, at least without a dynamical-collapse mechanism: in the Everett interpretation, it will yield an ontology where all the particles are completely delocalized. We only see a somewhat localized description of matter in three dimensions when we restrict to an Everett branch, and that restriction is normally motivated on functionalist grounds. But this may simply reflect subtleties of Ney’s intriguing proposal that I am not fully grasping at present.In summary, I congratulate Ney for a rich, subtle, and provocative account of wave function realism. Her edited collection of papers (coedited with Albert), The Wave Function, set the agenda for much of the metaphysics of quantum mechanics in the 2010s; The World in the Wave Function has a good chance of doing the same for the 2020s.