David Wallace argues that we should take quantum theory seriously as an account of what the world is like--which means accepting the idea that the universe is constantly branching into new universes. He presents an accessible but rigorous account of the 'Everett interpretation', the best way to make coherent sense of quantum physics.

The philosophy of biology should move to the center of the philosophy of science - a place it has not been accorded since the time of Mach. Physics was the paradigm of science, and its shadow falls across con temporary philosophy of biology as well, in a variety of contexts: reduction, organization and system, biochemical mechanism, and the models of law and explanation which derive from the Duhem-Popper Hempel tradition. This volume, we think, offers ample evidence of how good contempo (...) rary work in the philosophical understanding of biology has become. Marjorie Grene and Everett Mendelsohn aptly combine a deep philo sophical appreciation of conceptual issues in biology with an historical understanding of the radical changes in the science of biology since the 19th century. In this book, they present essays which probe such historical and methodological questions as reducibility, levels of organization, function and teleology, and the range of issues emerging from evolution ary theory and the species problem. In conjunction with Professor Grene's collection of essays on the philosophy of biology, The Under standing of Nature (Boston Studies in the Philosophy of Science, Vol. XXIII) and the occasional essays on these topics which we have published in other volumes (listed below), this volume contributes to bringing biology to the center of philosophical attention. Everett Mendelsohn, 'Explanation in Nineteenth Century Biology' (Boston Studies, Vol. II, 1965). David Hawkins, 'Taxonomy and Information', (Boston Studies, Vol. III, 1967). (shrink)

I address the problem of indefiniteness in quantum mechanics: the problem that the theory, without changes to its formalism, seems to predict that macroscopic quantities have no definite values. The Everett interpretation is often criticised along these lines, and I shall argue that much of this criticism rests on a false dichotomy: that the macroworld must either be written directly into the formalism or be regarded as somehow illusory. By means of analogy with other areas of physics, I develop the (...) view that the macroworld is instead to be understood in terms of certain structures and patterns which emerge from quantum theory (given appropriate dynamics, in particular decoherence). I extend this view to the observer, and in doing so make contact with functionalist theories of mind. (shrink)

The Everett interpretation of quantum mechanics - better known as the Many-Worlds Theory - has had a rather uneven reception. Mainstream philosophers have scarcely heard of it, save as science fiction. In philosophy of physics it is well known but has historically been fairly widely rejected. Among physicists, it is taken very seriously indeed, arguably tied for first place in popularity with more traditional operationalist views of quantum mechanics. In this article, I provide a fairly short and self-contained introduction to (...) the Everett interpretation as it is currently understood. I use little technical machinery, although I do assume the reader has encountered the measurement problem already. (shrink)

In recent years a number of authors sympathetic to Referentialistaccounts of proper names have argued that utterances containingempty names express `gappy,' or incomplete, propositions. In this paper I want to take issue with this suggestion.In particular, I argue versions of this approach developedby David Braun, Nathan Salmon, Ken Taylor, and by Fred Adams,Gary Fuller, and Robert Stecker.

The philosophical risks of politics -- Reflective simplification: the questions of a philosophical life -- Poverty eternal -- Life near the bone -- Wildness: the phenomenology of freedom -- How to mind your own business -- The fullness of life.

While rooted in careful study of Mead’s original writings and transcribed lectures and the historical context in which that work was carried out, the papers in this volume have brought Mead’s work to bear on contemporary issues in metaphysics, epistemology, cognitive science, and social and political philosophy.

This is a discussion of how we can understand the world-view given to us by the Everett interpretation of quantum mechanics, and in particular the role played by the concept of 'world'. The view presented is that we are entitled to use 'many-worlds' terminology even if the theory does not specify the worlds in the formalism; this is defended by means of an extensive analogy with the concept of an 'instant' or moment of time in relativity, with the lack of (...) a preferred foliation of spacetime being compared with the lack of a preferred basis in quantum theory. Implications for identity of worlds over time, and for relativistic quantum mechanics, are discussed. (shrink)

David Sosa, Michael Nelson, and Jason Stanley have recently offered a series of interesting and provocative challenges to Kripke's modal arguments against Descriptivism. In this paper I explore these challenges and some of the issues to which they give rise. I argue that, in the end, all three challenges fail.

An analysis is made of Deutsch's recent claim to have derived the Born rule from decision-theoretic assumptions. It is argued that Deutsch's proof must be understood in the explicit context of the Everett interpretation, and that in this context, it essentially succeeds. Some comments are made about the criticism of Deutsch's proof by Barnum, Caves, Finkelstein, Fuchs, and Schack; it is argued that the flaw which they point out in the proof does not apply if the Everett interpretation is assumed.

I consider exactly what is involved in a solution to the probability problem of the Everett interpretation, in the light of recent work on applying considerations from decision theory to that problem. I suggest an overall framework for understanding probability in a physical theory, and conclude that this framework, when applied to the Everett interpretation, yields the result that that interpretation satisfactorily solves the measurement problem. Introduction What is probability? 2.1 Objective probability and the Principal Principle 2.2 Three ways of (...) satisfying the functional definition 2.3 Cautious functionalism 2.4 Is the functional definition complete? The Everett interpretation and subjective uncertainty 3.1 Interpreting quantum mechanics 3.2 The need for subjective uncertainty 3.3 Saunders' argument for subjective uncertainty 3.4 Objections to Saunders' argument 3.5 Subjective uncertainty again: arguments from interpretative charity 3.6 Quantum weights and the functional definition of probability Rejecting subjective uncertainty 4.1 The fission program 4.2 Against the fission program Justifying the axioms of decision theory 5.1 The primitive status of the decision-theoretic axioms 5.2 Holistic scepticism 5.3 The role of an explanation of decision theory Conclusion. (shrink)

In this article, I briefly explain the quantum measurement problem and the Everett interpretation, in a way that is faithful to modern physics and yet accessible to readers without any physics training. I then consider the metaphysical lessons for ontology from quantum mechanics under the Everett interpretation. My conclusions are largely negative: I argue that very little can be said in full generality about the ontology of quantum mechanics, because quantum mechanics, like abstract classical mechanics, is a framework within which (...) we can consider different physical theories which have very little in common at the level of ontology. Along the way I discuss, and criticise, several positive ontological proposals that have been made in the context of the Everett interpretation: ontologies based on the so-called "eigenstate-eigenvalue link", ontologies based on taking the "many-worlds" language seriously at the fundamental level, and ontologies that treat the wavefunction as a complex field on a high-dimensional space. (shrink)

What would it mean to apply quantum theory, without restriction and without involving any notion of measurement and state reduction, to the whole universe? What would realism about the quantum state then imply? This book brings together an illustrious team of philosophers and physicists to debate these questions. The contributors broadly agree on the need, or aspiration, for a realist theory that unites micro- and macro-worlds. But they disagree on what this implies. Some argue that if unitary quantum evolution has (...) unrestricted application, and if the quantum state is taken to be something physically real, then this universe emerges from the quantum state as one of countless others, constantly branching in time, all of which are real. The result, they argue, is many worlds quantum theory, also known as the Everett interpretation of quantum mechanics. No other realist interpretation of unitary quantum theory has ever been found. Others argue in reply that this picture of many worlds is in no sense inherent to quantum theory, or fails to make physical sense, or is scientifically inadequate. The stuff of these worlds, what they are made of, is never adequately explained, nor are the worlds precisely defined; ordinary ideas about time and identity over time are compromised; no satisfactory role or substitute for probability can be found in many worlds theories; they can't explain experimental data; anyway, there are attractive realist alternatives to many worlds. Twenty original essays, accompanied by commentaries and discussions, examine these claims and counterclaims in depth. They consider questions of ontology - the existence of worlds; probability - whether and how probability can be related to the branching structure of the quantum state; alternatives to many worlds - whether there are one-world realist interpretations of quantum theory that leave quantum dynamics unchanged; and open questions even given many worlds, including the multiverse concept as it has arisen elsewhere in modern cosmology. A comprehensive introduction lays out the main arguments of the book, which provides a state-of-the-art guide to many worlds quantum theory and its problems. (shrink)

Sender–receiver games, first introduced by David Lewis ([1969]), have received increased attention in recent years as a formal model for the emergence of communication. Skyrms ([2010]) showed that simple models of reinforcement learning often succeed in forming efficient, albeit not necessarily minimal, signalling systems for a large family of games. Later, Alexander et al. ([2012]) showed that reinforcement learning, combined with forgetting, frequently produced both efficient and minimal signalling systems. In this article, I define a ‘dynamic’ sender–receiver game in (...) which the state–action pairs are not held constant over time and show that neither of these two models of learning learn to signal in this environment. However, a model of reinforcement learning with discounting of the past does learn to signal; it also gives rise to the phenomenon of linguistic drift. 1 Introduction2 Dynamic Signalling Games with Reinforcement Learning2.1 Introducing new states2.2 Swapping state–action pairs3 Discounting the Past3.1 Learning to signal in a dynamic world3.2 An unexpected outcome: linguistic drift4 ConclusionAppendix: A Markov Chain Analysis. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Philosophy of Music Education Review 11.2 (2003) 157-169 [Access article in PDF] Praxis and the PossibleThoughts on the Writings of Maxine Greene and Paulo Freire Randall Everett Allsup Columbia University Authors in a recent edition of the Philosophy of Music Education Review have assayed various understandings of praxis within the domain of music learning and teaching. 1 Leadened (perhaps) by history, this six-letter word sustains a multiplicity of meanings. (...) Philosophers as disparate as Aristotle, Marx and Engels, Sartre, Marshall Sahlins, Pierre Bourdieu, and David Elliott have represented praxis as a form of cultural-doing—but in many instances, this is where the similarities end. 2 By stirring up the pot, editor Estelle Jorgensen has sought to lighten history's burden, lest the word praxis suffer ossification, territorial over-use, or (worse) over-simplification.My first encounters with the word praxis were not particularly weighty or didactic. In graduate work with Maxine Greene, I would often hear praxis associated with the arts, with possibilities, openings, or alternatives. Greene does not construct a philosophical stance around the word; rather the conditions for praxis exist within the imagination, within "the capacity to look at things as if they could be otherwise." 3 Recalling Emily Dickinson, Greene writes that "imagination lights the possible's slow fuse." 4 Praxis, I think, is not simply the capacity to imagine alternative scenarios, but is instead the slow burning fuse of possibility and action. [End Page 157] This definition connects nicely with Paulo Freire, the revolutionary pedagogue who is of late often paired with Greene. 5 "Human activity," writes Freire, "consists of action and reflection: it is praxis." 6 The result is action upon our world.I must admit that as a long time fan of Freire (and as a student and friend of Greene), I have often wondered what exactly it means to act upon one's world, to name's one's world—especially as a music educator. Can such action lead to the creation of a love song? Can we say that certain art forms like the blues, be-bop, and jam band are more praxial than others? Might an encounter with Brahms lead to potentially transformative action upon my world?Freirian praxis, for sure, is non-neutral; it occurs when we disavow the traditional separation between abstracted learning and real life. After September 11, for example, my freshman music education majors created a requiem for the missing and the dead. This was the most difficult teaching I have ever facilitated. Before beginning our composition, we listened to several movements from the Faure Requiem, particularly the "In Paradisum": "May the choir of angels receive thee, and with Lazarus, once a beggar, mayst thou have eternal rest." We tried to attain Faure's sense of peace and hope; we admired his assurance that those who were gone were at rest, at peace. And we shared stories, and talked about our feelings and our pain. 7During this time, even the ordinary became infused with meaning. Banal pop music, like Enrique Inglesia's Hero, became a tribute song for many, including myself. Suddenly full of significance and relevance, I attended more closely to the simplicity of his chords and the sotto voce color of his voice: Would you die for the one you love? Hold me in your arms tonight. I wanna be your hero... The fact that this was an ordinary love song written some time before the terrorist attacks does not negate my feelings for it. Like Faure's requiem, it brought me a degree of comfort. In addition, I found solace in the music of Aaron Copland, particularly Quiet City. The open intervals seemed to speak of piece, quietude, and space. The opening trumpet recitative was defiant, but introspective—the English horn felt dark and mournful. I wanted to be back in an earlier New York, like the one that Copland knew and loved.Of course, acting upon our world requires inaction: thinking, perceiving, reflecting, reconceptualizing, connecting. For Freire and Greene, transformational or liberatory practice cannot... (shrink)

`Quantum theory' is not a single physical theory but a framework in which many different concrete theories fit. As such, a solution to the quantum measurement problem ought to provide a recipe to interpret each such concrete theory, in a mutually consistent way. But with the exception of the Everett interpretation, the mainextant solutions either try to make sense of the abstract framework as if it were concrete, or else interpret one particular quantum theory under the fiction that it is (...) fundamental and exact. In either case, these approaches are unable to help themselves to the very theory-laden, level-relative ways in which quantum theory makes contact with experiment in mainstream physics, and so are committed to major revisionary projects which have not been carried out even in outline. As such, only the Everett interpretation is currently suited to make sense of quantum physics as we find it. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Philosophy of Music Education Review 11.2 (2003) 157-169 [Access article in PDF] Praxis and the PossibleThoughts on the Writings of Maxine Greene and Paulo Freire Randall Everett Allsup Columbia University Authors in a recent edition of the Philosophy of Music Education Review have assayed various understandings of praxis within the domain of music learning and teaching. 1 Leadened (perhaps) by history, this six-letter word sustains a multiplicity of meanings. (...) Philosophers as disparate as Aristotle, Marx and Engels, Sartre, Marshall Sahlins, Pierre Bourdieu, and David Elliott have represented praxis as a form of cultural-doing—but in many instances, this is where the similarities end. 2 By stirring up the pot, editor Estelle Jorgensen has sought to lighten history's burden, lest the word praxis suffer ossification, territorial over-use, or (worse) over-simplification.My first encounters with the word praxis were not particularly weighty or didactic. In graduate work with Maxine Greene, I would often hear praxis associated with the arts, with possibilities, openings, or alternatives. Greene does not construct a philosophical stance around the word; rather the conditions for praxis exist within the imagination, within "the capacity to look at things as if they could be otherwise." 3 Recalling Emily Dickinson, Greene writes that "imagination lights the possible's slow fuse." 4 Praxis, I think, is not simply the capacity to imagine alternative scenarios, but is instead the slow burning fuse of possibility and action. [End Page 157] This definition connects nicely with Paulo Freire, the revolutionary pedagogue who is of late often paired with Greene. 5 "Human activity," writes Freire, "consists of action and reflection: it is praxis." 6 The result is action upon our world.I must admit that as a long time fan of Freire (and as a student and friend of Greene), I have often wondered what exactly it means to act upon one's world, to name's one's world—especially as a music educator. Can such action lead to the creation of a love song? Can we say that certain art forms like the blues, be-bop, and jam band are more praxial than others? Might an encounter with Brahms lead to potentially transformative action upon my world?Freirian praxis, for sure, is non-neutral; it occurs when we disavow the traditional separation between abstracted learning and real life. After September 11, for example, my freshman music education majors created a requiem for the missing and the dead. This was the most difficult teaching I have ever facilitated. Before beginning our composition, we listened to several movements from the Faure Requiem, particularly the "In Paradisum": "May the choir of angels receive thee, and with Lazarus, once a beggar, mayst thou have eternal rest." We tried to attain Faure's sense of peace and hope; we admired his assurance that those who were gone were at rest, at peace. And we shared stories, and talked about our feelings and our pain. 7During this time, even the ordinary became infused with meaning. Banal pop music, like Enrique Inglesia's Hero, became a tribute song for many, including myself. Suddenly full of significance and relevance, I attended more closely to the simplicity of his chords and the sotto voce color of his voice: Would you die for the one you love? Hold me in your arms tonight. I wanna be your hero... The fact that this was an ordinary love song written some time before the terrorist attacks does not negate my feelings for it. Like Faure's requiem, it brought me a degree of comfort. In addition, I found solace in the music of Aaron Copland, particularly Quiet City. The open intervals seemed to speak of piece, quietude, and space. The opening trumpet recitative was defiant, but introspective—the English horn felt dark and mournful. I wanted to be back in an earlier New York, like the one that Copland knew and loved.Of course, acting upon our world requires inaction: thinking, perceiving, reflecting, reconceptualizing, connecting. For Freire and Greene, transformational or liberatory practice cannot... (shrink)

Defenders of the Everettian version of quantum mechanics generally hold that it makes no difference to what we ought to do. This paper will argue against this stance, by considering the use of lotteries to select the recipients of indivisible goods. On orthodox non-Everettian metaphysics this practice faces the objection that only actual and not probable goods matter to distributive justice. However, this objection loses all force within Everettianism. This result should be of interest to both philosophers of physics and (...) to ethicists. (shrink)

The quantum theory of de Broglie and Bohm solves the measurement problem, but the hypothetical corpuscles play no role in the argument. The solution finds a more natural home in the Everett interpretation.

I make the case that the Universe according to unitary quantum theory has a branching structure, and so can literally be regarded as a "many-worlds" theory. These worlds are not part of the _fundamental_ ontology of quantum theory - instead, they are to be understood as structures, or patterns, emergent from the underlying theory, through the dynamical process of decoherence. That they are structures in this sense does not mean that they are in any way unreal: indeed, pretty much all (...) higher-level ontology in science, from tables to phonons to tigers, is likewise emergent. Unitary quantum theory is therefore a "many-worlds" theory without any modification of the mathematical structure of the theory: the Everett interpretation does not consist in adding worlds to the formalism, but in realising that they are there already. Our grounds for accepting the reality of those worlds is no more, but no less, than our grounds for accepting any other not-directly observable consequence of an empirically very successful theory. (shrink)

It seems to be widely assumed that the only effect of the Ghirardi-Rimini-Weber dynamical collapse mechanism on the `tails' of the wavefunction is to reduce their weight. In consequence it seems to be generally accepted that the tails behave exactly as do the various branches in the Everett interpretation except for their much lower weight. These assumptions are demonstrably inaccurate: the collapse mechanism has substantial and detectable effects within the tails. The relevance of this misconception for the dynamical-collapse theories is (...) debatable, though. (shrink)

I develop the decision-theoretic approach to quantum probability, originally proposed by David Deutsch, into a mathematically rigorous proof of the Born rule in (Everett-interpreted) quantum mechanics. I sketch the argument informally, then prove it formally, and lastly consider a number of proposed ``counter-examples'' to show exactly which premises of the argument they violate. (This is a preliminary version of a chapter to appear --- under the title ``How to prove the Born Rule'' --- in Saunders, Barrett, Kent and Wallace, (...) "Many worlds? Everett, quantum theory and reality", forthcoming from Oxford University Press.). (shrink)

Quantum theory plays an increasingly significant role in contemporary early-universe cosmology, most notably in the inflationary origins of the fluctuation spectrum of the microwave background radiation. I consider the two main strategies for interpreting standard quantum mechanics in the light of cosmology. I argue that the conceptual difficulties of the approaches based around an irreducible role for measurement - already very severe - become intolerable in a cosmological context, whereas the approach based around Everett's original idea of treating quantum systems (...) as closed systems handles cosmological quantum theory satisfactorily. Contemporary cosmology, which indeed applies standard quantum theory without supplementation or modification, is thus committed - tacitly or explictly - to the Everett interpretation. (shrink)

I present a proof of the quantum probability rule from decision-theoretic assumptions, in the context of the Everett interpretation. The basic ideas behind the proof are those presented in Deutsch's recent proof of the probability rule, but the proof is simpler and proceeds from weaker decision-theoretic assumptions. This makes it easier to discuss the conceptual ideas involved in the proof, and to show that they are defensible.

An extended analysis is given of the program, originally suggested by Deutsch, of solving the probability problem in the Everett interpretation by means of decision theory. Deutsch's own proof is discussed, and alternatives are presented which are based upon different decision theories and upon Gleason's Theorem. It is argued that decision theory gives Everettians most or all of what they need from `probability'. Contact is made with Lewis's Principal Principle linking subjective credence with objective chance: an Everettian Principal Principle is (...) formulated, and shown to be at least as defensible as the usual Principle. Some consequences of (Everettian) quantum mechanics for decision theory itself are also discussed. -/- [NB: this this long (70 pages) and occasionally rambling online-only paper has been almost entirely superseded by material in subsequent; if something is not included in them it usually means that I have had second thoughts. I include it for completeness only. (shrink)

NGC 1300 (shown in figure 1) is a spiral galaxy 65 million light years from Earth.1 We have never been there, and (although I would love to be wrong about this) we will never go there; all we will ever know about NGC 1300 is what we can see of it from sixty-five million light years away, and what we can infer from our best physics. Fortunately, “what we can infer from our best physics” is actually quite a lot. To (...) take a particular example: our best theory of galaxies tells us that that hazy glow is actually made up of the light of hundreds of billions of stars; our best theories of planetary formation tell us that a sizable fraction of those stars.. (shrink)

I criticize the widely-defended view that the quantum measurement problem is an example of underdetermination of theory by evidence: more specifically, the view that the unmodified, unitary quantum formalism (interpreted following Everett) is empirically indistinguishable from Bohmian Mechanics and from dynamical-collapse theories like the GRW or CSL theories. I argue that there as yet no empirically successful generalization of either theory to interacting quantum field theory and so the apparent underdetermination is broken by a very large class of quantum experiments (...) that require field theory somewhere in their description. The class of quantum experiments reproducible by either is much smaller than is commonly recognized and excludes many of the most iconic successes of quantum mechanics, including the quantitative account of Rayleigh scattering that explains the color of the sky. I respond to various arguments to the contrary in the recent literature. (shrink)

The decision-theoretic account of probability in the Everett or many-worlds interpretation, advanced by David Deutsch and David Wallace, is shown to be circular. Talk of probability in Everett presumes the existence of a preferred basis to identify measurement outcomes for the probabilities to range over. But the existence of a preferred basis can only be established by the process of decoherence, which is itself probabilistic.

The decision-theoretic account of probability in the Everett or many-worlds interpretation, advanced by David Deutsch and David Wallace, is shown to be circular. Talk of probability in Everett presumes the existence of a preferred basis to identify measurement outcomes for the probabilities to range over. But the existence of a preferred basis can only be established by the process of decoherence, which is itself probabilistic.

The authors consider a recurring objection to fictional realism, the view that fictional characters are objects. The authors call this the counting objection. Russell presses a version of the objection against Meinong’s view. Everett presses a version of the objection against contemporary fictional realist views, as do Nolan and Sandgren. As the authors see it, the objection assumes that the fictional realist must provide criteria of identity for fictional characters, so its force depends on the plausibility of that assumption. Rather (...) than coming up with such criteria, a fictional realist might argue that the demand is misplaced. (shrink)