Alastair Wilson

This chapter introduces the overall aims and themes of this volume and relates them to the individual chapters. It first presents and compares different conceptions of levels of explanation which have figured in philosophy of science and in philosophy more broadly, distinguishing explanatory levels from related accounts of levels including compositional levels and levels of organization. It then identifies some themes which unite the chapters of the volume: the function of explanatory levels in scientific and philosophical reasoning, the implications of using models which span different explanatory levels, and the prospects for different levels of explanation of the phenomenon of levels of explanation itself. It concludes with summaries of the individual chapters, highlighting their connections to the broader themes of the volume.

Do causes always precede their effects? Is causation across a temporal gap possible? Is simultaneous causation possible? The comparative neglect of such questions means that we still lack a clear view of the underlying nature of causation. Metaphysicians typically distinguish sharply between grounding and causation, and philosophers of science typically distinguish sharply between causal and non-causal explanation, but there has been surprisingly little discussion of how exactly to draw these distinctions. This chapter argues that six of the most obvious criteria fail to capture the intended distinction between causation and grounding. The chapter proposes and defends an alternative criterion in terms of the principles mediating the dependency, and explores some of the implications of this criterion for the possibility of simultaneous causation in physics.

Various approaches to quantum gravity render spacetime an emergent phenomenon, with the existence and properties of spacetime depending on a non-spatiotemporal underlying reality. This chapter investigates the mode of dependence that is involved. I explain and defend my recent proposal to classify different kinds of dependencies in terms of the principles mediating the dependency, and apply this proposal to the emergence of spacetime. While philosophers have typically interpreted spacetime emergence as involving metaphysical grounding, I show how premises that are widely endorsed lead us to classify the emergence of spacetime in loop quantum gravity as causal in nature. I recommend spacetime functionalism as a resolution of this puzzle that vindicates the natural view of spacetime emergence as non-causal. I then explore a different approach to spacetime emergence in quantum gravity, the ‘many-instant landscape’ scenario described by Gomes, and show how it fits into the proposed framework for spacetime emergence.

The most familiar philosophical conception of objective chance renders determinism incompatible with non-trivial chances. This conception—associated in particular with the work of David Lewis—is not a good fit with use of the word ‘chance’ and cognates in ordinary discourse. A generalized framework for chance reconciles determinism with non-trivial chances, and provides for a more charitable interpretation of ordinary chance-talk. Variation in an admissible ‘evidence base’ generates a spectrum of different chance functions: coarse-grainings of the evidence base generate a partial ordering of chance functions, with finer trumping coarser if known. A contextual mechanism is proposed, according to which users of chance-talk refer to different chance functions in different contexts. Admissible evidence is identified with available evidence: evidence which could be obtained. Consequently, attributions of objective chances inherit the relatively familiar context-sensitivity associated with the modal ‘could’. This context-dependency undermines certain arguments for the incompatibility of chance with determinism.

Laws of nature have two characteristic features. They are general, in that they apply across all situations of a given kind – although they are typically restricted to particular domains. They are also modal, in that they apply across possible situations as well as actual situations. This simple account captures the core features of laws and their differences across distinct fields, and it helps to explain why laws are less prominent in some fields than in others. The most fundamental laws of physics are a special case, in that they are maximally general: they apply to all possible situations whatsoever. This provides a principled basis for a reductionist – or, to use a softer term, physicalist – view of nature. Any plausible reductionism, however, still recognizes a rich world of explanations beyond physics. In domains such as biology where laws retain important explanatory power, as well as in more anarchic domains such as history, physics is not and can never tell the whole story – and physics itself is part of the explanation for why that is so.

How do we know what is possible or impossible, what is inevitable or unattainable, or what would happen under which circumstances? Since modal facts seem distinctively mysterious and difficult to know, the epistemology of modality has historically been fraught with uncertainty and disagreement. The recent literature has been dominated by rationalist approaches that emphasise a priori reasoning (sometimes including direct intuition of possibility). Only recently have alternative approaches emerged which recognize a broader range of sources of modal knowledge. Yet even emerging non-rationalist views have tended to assign scientific investigation at best a supporting role. Our project in this Element is to develop and defend a new approach to the epistemology of modal facts which assigns a central role to scientific investigation. According to modal naturalism, science (construed broadly) is our primary source of evidence concerning the modal facts.

How, if at all, can scientific progress improve our view of the modal facts? According to rationalist approaches to modal epistemology, science has no substantive role: a priori reflection reveals the structure of modal space, and a posteriori science merely locates us within that modal space, by identifying the actual properties and structures instantiated at our world. According to modal naturalist approaches, science provides evidence about the structure of the underlying modal space. In this paper I distinguish four versions of modal naturalism, with science playing an increasingly robust evidential role, and discuss their plausibility.

The different sciences furnish us with a wide variety of explanations: some work at macroscopic scales, some work at microscopic scales, and some operate across different levels. How do these different explanatory levels relate to one another, and what is an explanatory level in the first place? Over the last 50 years, more and more philosophers—both reductionists and anti-reductionists—no longer subscribe to the idea that the best explanation resides at the fundamental physical level. New challenges arise from the success of scientific explanations employing multi-level models which mix levels of explanation, from distinctive differences between levels structures in biology, cognitive science, and social science, from the apparently radical reimagining of the explanatory role of spacetime in our current best theories of fundamental physics, and from the enduring mystery of how higher-level explanations are possible in the first place. These questions naturally connect to classic philosophical ways of thinking about the relationships between levels: reduction, emergence, and fundamentality. This volume presents a snapshot of cutting-edge research on explanatory levels, from their conceptual foundations to the details of how they are used in scientific practice.

When once-successful physical theories are abandoned, common wisdom has it that their characteristic theoretical entities are abandoned with them: examples include phlogiston, light rays, Newtonian forces, Euclidean space. But sometimes a theory sees ongoing use, despite being superseded. What should scientific realists say about the characteristic entities of the theories in such cases? The standard answer is that these ‘theoretical relicts’ are merely useful fictions. In this paper we offer a different answer. We start by distinguishing horizontal reduction (in which a superseded theory approximates the successor theory) from vertical reduction (in which a higher-level theory abstracts away from the lower-level theory, but nonetheless can be constructed from it); these are usually regarded as having different ontological consequences. We describe a ‘verticalization’ procedure which transforms horizontal reductions into vertical reductions. The resulting verticalized theories are abstractions rather than approximations, with restricted domains. We identify a sense in which the higher-level theory describes distinct subject matters from the lower-level theory, enabling in certain cases the higher-level theory to retain distinctive explanatory power even in the presence of reduction. We suggest that theoretical entities from superseded theories should be retained in a scientific realist worldview just when, reinterpreted as higher-level abstractions, those theories and their characteristic entities continue to perform distinctive explanatory work in providing the best explanation for less-fundamental phenomena of interest. In slogan form: a good relict is an emergent relict.

My topic in this paper is the relationships of metaphysical priority which might hold between the different alethic modal statuses—necessity, contingency, possibility and impossibility. In particular, I am interested in exploring the view that the necessity of necessities is ungrounded while the contingency of contingencies is grounded—a scenario I call ‘necessity first’. I will explicate and scrutinize the contrast between necessity first and its ‘contingency first’ contrary, and then compare both views with ‘multimodal’ and ‘amodal’ alternatives, drawing on David Lewis’s modal realism and Barbara Vetter’s potentialism as example cases. I will then defend the necessity-first point of view from a reversed version of Blackburn’s classic dilemma against theories of the source of necessity.

Distinctions in fundamentality between different levels of description are central to the viability of contemporary decoherence-based Everettian quantum mechanics (EQM). This approach to quantum theory characteristically combines a determinate fundamental reality (one universal wave function) with an indeterminate emergent reality (multiple decoherent worlds). In this chapter I explore how the Everettian appeal to fundamentality and emergence can be understood within existing metaphysical frameworks, identify grounding and concept fundamentality as promising theoretical tools, and use them to characterize a system of explanatory levels (with associated laws of nature) for EQM. This Everettian level structure encompasses and extends the ‘classical’ levels structure. The ‘classical’ levels of physics, chemistry, biology, etc. are recovered, but they are emergent in character and potentially variable across Everett worlds. EQM invokes an additional fundamental level, not present in the classical levels picture, and a novel potential role for self-location in interlevel metaphysics. When given a modal realist interpretation, EQM also makes trouble for supervenience-based approaches to levels.

In On the Plurality of Worlds (Lewis 1986), David Lewis imposes a condition on realist theories of modality which he calls ‘plenitude’. Lewis apparently assigns this condition considerable importance, and uses it to motivate his Humean principle of recombination, but he never says exactly what plenitude amounts to. This chapter first sets aside some obvious ways of reconstructing the plenitude criterion which do not fit with the textual evidence. An objection to modal realism due to John Divers and Joseph Melia (Divers and Melia 2002) is diagnosed as equivocating between an overly-demanding plenitude constraint and a weaker constraint which fails to establish their conclusion. An alternative deflationary interpretation of the plenitude condition has it following from an application of standard theoretical virtues to a modal realist’s total theory; Lewis’ correspondence provides new evidence in support of this interpretation. The deflationary plenitude criterion also has broader application, beyond Lewisian modal realism.

This article explores three ways in which physics may involve counterpossible reasoning. The first way arises when evaluating false theories: to say what the world would be like if the theory were true, we need to evaluate counterfactuals with physically impossible antecedents. The second way relates to the role of counterfactuals in characterizing causal structure: to say what causes what in physics, we need to make reference to physically impossible scenarios. The third way is novel: to model metaphysical dependence in physics, we need to consider counterfactual consequences of metaphysical impossibilities. Physics accordingly bears substantial and surprising counterpossible commitments.

What explains the outcomes of chance processes? We claim that their setups do. Chances, we think, mediate these explanations of outcome by setup but do not feature in them. Facts about chances do feature in explanations of a different kind: higher-order explanations, which explain how and why setups explain their outcomes. In this paper, we elucidate this 'mediator view' of chancy explanation and defend it from a series of objections. We then show how it changes the playing field in four metaphysical disputes concerning chance. First, it makes it more plausible that even low chances can have explanatory power. Second, it undercuts a circularity objection against reductionist theories of chance. Third, it redirects the debate about a prominent argument against epistemic theories of chance. Finally, it sheds light on potential chancy explanations of the Universe's origin.

The Routledge Companion to Philosophy of Physics is a comprehensive and authoritative guide to the state of the art in the philosophy of physics. It contains 54 self-contained chapters written by leading philosophers of physics at both senior and junior levels, making it the most thorough and detailed volume of its type on the market – nearly every major perspective in the field is represented. The Companion’s 54 chapters are organized into 12 sections. The first seven sections cover all of the major physical theories investigated by philosophers of physics today, and the last five explore key themes that unite the study of these theories. I. Classical Mechanics II. Special Relativity III. General Relativity IV. Non-Relativistic Quantum Mechanics V. Quantum Field Theory VI. Quantum Gravity VII. Statistical Mechanics and Thermodynamics VIII. Explanation IX. Intertheoretic Relations X. Symmetries XI. Metaphysics XII. Cosmology

This book defends a radical new theory of contingency as a physical phenomenon. Drawing on the many-worlds approach to quantum theory and cutting-edge metaphysics and philosophy of science, it argues that quantum theories are best understood as telling us about the space of genuine possibilities, rather than as telling us solely about actuality. When quantum physics is taken seriously in the way first proposed by Hugh Everett III, it provides the resources for a new systematic metaphysical framework encompassing possibility, necessity, actuality, chance, counterfactuals, and a host of related modal notions. Rationalist metaphysicians argue that the metaphysics of modality is strictly prior to any scientific investigation; metaphysics establishes which worlds are possible, and physics merely checks which of these worlds is actual. Naturalistic metaphysicians respond that science may discover new possibilities and new impossibilities. This book's quantum theory of contingency takes naturalistic metaphysics one step further, allowing that science may discover what it is to be possible. As electromagnetism revealed the nature of light, as acoustics revealed the nature of sound, as statistical mechanics revealed the nature of heat, so quantum physics reveals the nature of contingency.

There is much to admire in this book. As a rigorous and systematic physics-oriented presentation of an austere empiricist fundamental metaphysics, it has no real rivals. The clarity with which the overall vision is presented will provide a valuable stalking-horse for those who would defend less austere approaches in the future. Esfeld and Deckert never shy away from the radical consequences of their approach, or try to disguise its revisionary nature. I also found several points of agreement with Esfeld and Deckert’s metaphysical outlook. In particular, I thought their form of structural realism sophisticated and plausible and their application of it to contemporary physics salutary. Fundamental metaphysics would be a more respectable discipline if all of its exponents felt the need to show how their preferred ontology plays out in the context of real physics. These points of agreement noted, I will concentrate in these comments on points where I disagree with Esfeld and Deckert. From my perspective, the metaphysical project of the book is subject to two serious objections: firstly, it remains insufficiently naturalistic and overly a prioristic, and secondly, it drains much of the explanatory power out of fundamental physics.

In his book Objective Becoming (Skow 2015), Bradford Skow has offered a rich and systematic treatment of the passage of time. We learn much about what objective passage could and could not amount to from engaging with his careful work. Skow’s overall conclusion is that the ‘block universe’ deflationary theory of passage is stronger than any currently available version of the recently-popular moving spotlight theory of temporal passage. To help establish this conclusion, Skow provides a taxonomy of theories of temporal passage, including some new and unfamiliar theories that he classifies as versions of the moving spotlight view. These new theories are worthy of further study; however, in this commentary I will argue that they suffer from some problems that render them uncongenial to committed believers in temporal passage (Sections 2–5). I’ll also make a few brief remarks about some other arguments in the book, concerning branching time (Section 6), relativistic presentness (Section 7), and temporal experience (Section 8).

Dispositions can combine as vector sums. Recent authors on dispositions, such as George Molnar and Stephen Mumford, have responded to this feature of dispositions by introducing a distinction between effects and contributions to effects, and by identifying disposition-manifestations with the latter. But some have been sceptical of the reality or knowability of component vectors; Jennifer McKitrick (forthcoming) presses these concerns against the conception of manifestations as contributions to effects. In this paper, I aim to respond to McKitrick's arguments and to defend the metaphysical and epistemological propriety of component vectors. My strategy appeals to varying kinematic frames of reference. By transforming to the appropriate non-inertial frame, component acceleration vectors can be transformed into resultant acceleration vectors, and in such frames they become directly observable. Being a component acceleration vector and being a resultant acceleration vector are both frame-dependent properties of properties; they are not to be thought of as intrinsic or fundamental properties of an acceleration vector, but as artefacts of our frame-dependent notation for representing vector quantities. To conclude the paper, I defend the view proposed against two styles of objection. The first objection resurrects scepticism about component vectors as scepticism about fundamental component vectors. The second objection questions the need for reference frames in the explanation by invoking a 'counterfactual' theory of contributions.

If the most familiar overlapping interpretation of Everettian quantum mechanics is correct, then each of us is constantly splitting into multiple people. This consequence gives rise to the quantum doomsday argument, which threatens to draw crippling epistemic consequences from EQM. However, a diverging interpretation of EQM undermines the quantum doomsday argument completely. This appears to tell in favour of the diverging interpretation. But it is surprising that a metaphysical question that is apparently underdetermined by the physics should be settled by purely epistemological considerations; and I argue that the positive case for divergence based on the quantum doomsday effect is ultimately unsuccessful. I discuss how some influential treatments of Everettian confirmation handle the quantum doomsday puzzle, and suggest that it can most satisfyingly be resolved via a naturalistic approach to the metaphysics of modality.

This volume is a collection of cutting-edge research papers in scientifically informed metaphysics, tackling a range of philosophical puzzles which have emerged from recent work on chance and temporal asymmetry. How do the probabilities found in fundamental physics and the probabilities of the special sciences relate to one another? How can we account for the normative significance of chance? Can constraints on the initial conditions of the universe underwrite the second law of thermodynamics, and potentially also all other lawlike regularities? How does contemporary quantum theory bear on debates over the nature of chance and the arrow of time? What grounds do we have for believing in a fundamental temporal direction, or flow? And how do all these questions connect up with one another? The aim of the volume is both to survey and summarize recent debates about chance and temporal asymmetry and to push them forward. The authors bring perspectives from metaphysics, from philosophy of physics and from philosophy of probability. Mainstream approaches are subjected to searching new critiques, and bold new proposals are made concerning (inter alia) the semantics of chance-attributions, the justification of the Principal Principle connecting chance and degree of belief, the limits on inter-theoretic reduction and the source of the temporal asymmetry of human experience.

The most familiar philosophical conception of objective chance renders determinism incompatible with non-trivial chances. This conception – associated in particular with the work of David Lewis – is not a good fit with our use of the word ‘chance’ and its cognates in ordinary discourse. In this paper we show how a generalized framework for chance can reconcile determinism with non-trivial chances, and provide for a more charitable interpretation of ordinary chance-talk. According to our proposal, variation in an admissible ‘evidence base’ generates a spectrum of different chance functions. Successive coarse-grainings of the evidence base generates a partial ordering of chance functions, with finer trumping coarser if known. We suggest that chance-attributions in ordinary discourse express different chance functions in different contexts, and we sketch a potential contextual mechanism for making particular chance functions salient. The mechanism involves the idea that admissible evidence is available evidence: propositions that could be known. A consequence is that attributions of objective chances inherit the relatively familiar context-sensitivity associated with the modal ‘could’. We show how this context-dependency undermines certain arguments for the incompatibility of chance with determinism.

Darren Bradley has recently appealed to observation selection effects to argue that conditionalization presents no special problem for Everettian quantum mechanics, and to defend the ‘halfer’ answer to the puzzle of Sleeping Beauty. I assess Bradley’s arguments and conclude that while he is right about confirmation in Everettian quantum mechanics, he is wrong about Sleeping Beauty. This result is doubly good news for Everettians: they can endorse Bayesian confirmation theory without qualification, but they are not thereby compelled to adopt the unpopular ‘halfer’ answer in Sleeping Beauty. These considerations suggest that objective chance is playing an important and under-appreciated role in Sleeping Beauty.

Darren Bradley has recently appealed to observation selection effects to argue that conditionalization presents no special problem for Everettian quantum mechanics, and to defend the ‘halfer’ answer to the puzzle of Sleeping Beauty. I assess Bradley’s arguments and conclude that while he is right about confirmation in Everettian quantum mechanics, he is wrong about Sleeping Beauty. This result is doubly good news for Everettians: they can endorse Bayesian confirmation theory without qualification, but they are not thereby compelled to adopt the unpopular ‘halfer’ answer in Sleeping Beauty. These considerations suggest that objective chance is playing an important and under-appreciated role in Sleeping Beauty. 1 Introduction2 Confirmation in Everettian Quantum Mechanics3 Sleeping Beauty4 The Selection Model5 Bradley’s Argument6 The Right Route to ⅓7 The Breakdown of the Analogy8 Alternative Diagnoses9 God’s Gambling Game10 Non-chancy Sleeping Beauty Cases11 Conclusion