Jessica Wilson’s Metaphysical Emergence is the product of two decades of thought, worked out in more than two dozen articles. In it, she seeks to understand the way(s) that a variety of complex natural entities (whether objects, properties, states, events, or processes) manifest dependence with autonomy in relation to constituting or otherwise sustaining less complex entities. Wilson provides a detailed, well-conceived, and clarifying taxonomy of competing approaches that divides them into two families, weak and strong. Unlike many others, Wilson is alive to the possibility that we may need accounts from each family to understand all instances of emergence in the natural world. In arguing meticulously for her favored accounts of each kind and their most plausible domains of application, she engages with equal depth a wide swathe of metaphysical and scientific theorizing, both contemporary and (for methodological lessons) historical. The book is masterful. While it will prove challenging to the uninitiated reader, patient study of it will yield a clear and comprehensive understanding of a large and at times confusing philosophical and scientific literature. At the same time, the author forcefully defends a distinctive and attractive perspective that is likely to be at the center of discussion in the coming years.Wilson begins by observing that the physical and life sciences constitute an interwoven hierarchy. Our best theories treat readily observable phenomena as composed of more basic phenomena, themselves similarly composed, in multiple iterations. At the same time, composed entities of many kinds are in the domain of successful special sciences that posit distinctive properties and causal-explanatory principles (as does ordinary experience in certain cases, described below). This gives prima facie plausibility to the general view that the entities these special sciences (and certain experiences) characterize are not ontologically reducible to entities described in any of the more basic sciences. As Wilson frames it, we should adopt the methodological criterion that an adequate account will “make natural (straightforward, default) and realistic sense of the appearances of metaphysical emergence, in the absence of specific reasons to think that this cannot be done” (36). In other words, Wilson does not set herself the goal of defeating the determined reductionist, who gives outsized weight to ontological parsimony, on her own terms. (This criterion is deployed in setting aside the reductionist stances of Alyssa Ney and John Heil [78]; it wasn’t clear to me that it was consistently honored, however, when she was criticizing the weak emergentist accounts rival to her own offered by Mark Bedau [169] and Sandra Mitchell [172].)Wilson takes seriously the charge that there is reason beyond mere love of parsimony to think that a realistic account of emergence cannot be sustained. Indeed, she frames her constructive exploration of theories of weak and strong emergence as responses to the ‘overdetermination’ problem of higher-level causation, pressed by Jaegwon Kim in many writings from 1989 forward (39–46). This problematic assumes that every microphysical effect has a sufficient microphysical cause and that, unusual cases aside, physical effects do not in general have distinct, cotemporal causes that are each individually sufficient (41). The problem boils down to its being a plausible implication of macro-micro dependency that macro causes (if such there be) will always have ‘downward’ micro (as well as same-level macro) effects, which is inconsistent with the conjunction of the two assumptions.Against this background challenge, weak emergence accounts deny the ‘no overdetermination’ assumption, while strong emergence accounts deny ‘causal closure’—that is, that micro effects always have sufficient micro causes. Weak emergence is thus consistent with physicalism, while strong emergence is not. Wilson argues that the many proposed accounts of weak emergence all adhere to the following schema:Weak Emergence: What it is for a token feature S to be Weakly metaphysically emergent from token feature P on a given occasion is for it to be the case, on that occasion, (i) that S cotemporally materially depends on P, and (ii) that S has a non-empty proper subset of the token powers had by P (74).Clause (ii) ensures S’s distinctive efficaciousness vis-à-vis P by subtraction: it potentially contributes to fewer effects. Wilson’s corresponding schema for strong emergence replaces (ii) with the requirement that S has at least one token power not identical with any token power of P. Here distinctness comes through addition: a strongly emergent feature potentially contributes to more effects than its base feature.With these schematic understandings in hand, Wilson devotes chapters 3 and 4 to responding to a range of arguments against the viability of weak and strong emergence, so understood. Some arguments challenge the sufficiency of the schemas for securing autonomy; others challenge their claim to entail physicalism or its denial; and still others challenge their necessity for securing autonomy—that is, they deny that Wilson’s subset/additional token power conditions are necessary to characterize weak/strong autonomy. Wilson also here signals her two preferred ways to instantiate the weak emergence schema. On the first, weak emergents are determinables of lower-level realizers; on the second, weak emergents arise when composing entities are constrained in such a way that fewer degrees of freedom (in dependent parameters) are required to characterize the emergent entity than are needed to characterize the state of the system of entities on which it depends (crudely put: many microscopic details are irrelevant to characterizing the macroscopic state and behavior).These chapters are followed by a chapter discussing the emergence status of complex systems, whose general characterization is disputed, but which include such phenomena as turbulent water flows, phase transitions, weather patterns, flocking behavior, and artificial cellular automata including John Conway’s ‘Game of Life’ (155). Wilson observes that such phenomena were once taken to be prime candidates for strong emergence whereas they now are widely seen as weakly emergent only, at least by philosophers (many scientists are notoriously difficult to interpret when speaking on this matter). She crisply summarizes and at points criticizes the philosophical glosses on select phenomena in this area by some recent philosophers of science. This chapter is usefully grouped with the preceding two because in it Wilson develops at length one of her own preferred ways to flesh out the weak emergence schema, in terms of reduced degrees of freedom (177–80). Taken together, chapters 3 through 5 constitute the heart of the book, enabling the reader to appreciate the motivation for and power of her schematic and instantiated proposals, particularly the initially surprising suggestion that weak emergence may be secured via subtraction.The final three chapters (apart from a short postlude) discuss other possible sources of emergence that indirectly or directly may point to strong emergence, and it is here that I will focus my critical remarks. Chapter 6 considers the status of ‘ordinary (inanimate) objects’, both natural and artifactual. Wilson argues, first, that many ordinary objects fall in the domain of classical mechanics, and classical mechanics is properly seen as a special science; second, that many ordinary objects have functionally specified persistence conditions that are insensitive to precise micro-configurations. Both of these considerations are amenable to the power subset treatment that is the hallmark of weak emergence. (She does not consider the worry that some objects fall under both categories and these may yield conflicting judgments: consider from the alternating perspectives of classical mechanics and human cultural judgment the fate of a clay statue that undergoes deformation into a spherical lump.) Wilson then notes that ordinary objects typically have imprecise boundaries. Given her realism concerning weak emergents, this leads her to embrace metaphysical indeterminacy, which she explicates as cases where an entity has a determinable property but no unique determinate (something commonly ruled out by analyses of the determinable-determinate relation). She contends that in these cases there are a plurality of heavily overlapping ‘micro-configurations’ (where this expression perhaps refers plurally rather than singularly, but Wilson is not explicit) that do possess the corresponding determinates, and on this basis calls these cases ones of ‘glutty’ rather than ‘gappy’ indeterminacy. She suggests, in accord with her general view, that here, too, there are fewer associated powers than are had by the corresponding determinates, which appears to suggest that the effects associated with an object’s indeterminate boundaries will themselves be indeterminate to an extent—thereby widening the scope of metaphysical indeterminacy. (And note again the possibility that classical mechanical and human cultural perspectives may yield differing judgments on the range of indeterminacy. If so, which is correct?) Finally, Wilson makes the interesting and original observation that, if certain human mental features are strongly emergent, then intentional stipulation of functional roles upon artifacts may imply that the latter have emergent powers, insofar as they are able to produce, for example, aesthetic and economic responses in us.Chapter 7 considers human and animal consciousness and argues that the knowledge and conceivability arguments for its strong emergence do not succeed, leaving the strong emergence of consciousness an open empirical question, while its weak emergence is best secured through recognizing that qualitative states of consciousness are typically not maximally determinate. Her analysis of the ‘zombie’-style conceivability argument is too complex to consider here; it is rooted in her defense of an alternative account to two-dimensional semantics for determining the intensions of terms. I found her rebuttal of the knowledge argument, by contrast, uncharacteristically unclear. I believe that part of what drives it is her ontology of weak emergence, which, while allowing that all the microphysical entities collectively possess all the natural powers there are, denies that they ‘encode’ all the properties and associated patterns there are. Fair enough, but what is at issue in the knowledge argument is whether there are basic intrinsic properties of experiences that are necessarily outside the domain of future functional-cognitive neuroscience. I don’t see anything in Wilson’s remarks (220–25) that shows how a physicalist could happily allow that there are.Chapter 8 then takes up the nature and existence of human free will. Wilson defends the interesting claim that there are strong parallels between the problem of mental causation and the traditional problem of free will. While I agree that there are instructive cross-lessons to be had, the particular way she sets up the problems, in terms of responses to a material conditional, misrepresents certain positions in the free will debate, which have to be understood in modal terms. Furthermore, not all the varieties of compatibilism, including one she discusses, are properly represented as conceiving free will as a matter of one’s choices not being determined by a proper subset of antecedent conditions. Wilson then suggests, plausibly enough, that philosophers who believe we exercise an indeterministic form of agency (‘libertarians’) are likely to be drawn to thinking such agency is strongly emergent. Finally, she argues that we have good reason to believe that this is so. Her argument has three premises: we experience ourselves as exercising causal-law transcending freedom of choice; we are entitled to take such experience as veridical, absent good reason to think it is mistaken; there are not good reasons to think it mistaken (277). This is a familiar line of argument. The epistemic principle concerning defeasible trust in one’s experiences is widely accepted. Wilson confines her defense of the ‘no empirical defeaters’ premise to a rebuttal of the Libet brain-activity-preceding-conscious-awareness-of-choice studies. As many have argued, these studies hardly establish what Benjamin Libet and others claimed. But a libertarian free will skeptic is apt to point to other kinds of studies indicating limited awareness of internal and external influences on decision-making—as well as to a general argument that the (supposed) absence of strong emergence anywhere else in nature makes it implausible that it should occur in just this one particular instance. It would have been good to see this widespread attitude discussed in such a comprehensive book on emergence. Consider, finally, Wilson’s first premise that we experience ourselves as exercising causal-law transcending freedom of choice. This is a fairly popular stance, but it is not generally true, as Wilson claims, that “even non-libertarians agree that we seem to freely choose, in ways transcending any nomological net” (278). Many compatibilists, historical and contemporary, have disputed this. There is also now a small literature on the experience of agency, both empirical and philosophical. There doesn’t as yet appear to be a consensus view with respect to the specific contention on which Wilson’s argument rests.As the above should make plain, this book is packed with a rich array of ideas and arguments. For a comprehensive, clear, and systematic treatment of the topic of emergence that fairly represents the range and content of views while powerfully defending the author’s own (and attractive) perspective, the reader can do no better than to carefully work through Wilson’s book.