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)

GRW theory offers precise laws for the collapse of the wave function. These collapses are characterized by two new constants, \ and \ . Recent work has put experimental upper bounds on the collapse rate, \ . Lower bounds on \ have been more controversial since GRW begins to take on a many-worlds character for small values of \ . Here I examine GRW in this odd region of parameter space where collapse events act as natural disasters that destroy branches (...) of the wave function along with their occupants. Our continued survival provides evidence that we don’t live in a universe like that. I offer a quantitative analysis of how such evidence can be used to assess versions of GRW with small collapse rates in an effort to move towards more principled and experimentally-informed lower bounds for \. (shrink)

After rejecting substance dualism, some naturalists embrace patternism. It states that persons are bodies and that bodies are material machines running abstract person programs. Following Aristotle, these person programs are souls. Patternists adopt four-dimensionalist theories of persistence: Bodies are 3D stages of 4D lives. Patternism permits at least six types of life after death. It permits quantum immortality, teleportation, salvation through advanced technology, promotion out of a simulated reality, computational monadology, and the revision theory of resurrection.

How should our beliefs change over time? The standard answer to this question is the Bayesian one. But while the Bayesian account works well with respect to beliefs about the world, it breaks down when applied to self-locating or de se beliefs. In this work I explore ways to extend Bayesianism in order to accommodate de se beliefs. I begin by assessing, and ultimately rejecting, attempts to resolve these issues by appealing to Dutch books and chance-credence principles. I then propose (...) and examine several accounts of the dynamics of de se beliefs. These examinations suggest that an extension of Bayesianism to de se beliefs will require some uncomfortable choices. I conclude by laying out the options available, and assessing the prospects of each. (shrink)

Lewis is famous as a contemporary philosophical system-builder. The most obvious way his philosophy exhibited a system was in its content: Lewis’s metaphysics, for example, provided answers to many metaphysical puzzles in an integrated way, and there are illuminating connections to be drawn between his general metaphysical views and, for example, his various views about the mind and its place in nature.

The question as to whether some objects are possible worlds that have an initial segment in common, i.e. so that their fusion is a temporal tree whose branches are possible worlds, arises both for those who hold that our universe has the structure of a temporal tree and for those who hold that what there is includes concrete universes of every possible variety. The notion of “possible world” employed in the question is seen to be the notion of an object (...) of a kind such that objects of that kind play a certain theoretical role. Lewis’s discussion of the question is thereby clarified but is nevertheless inadequate; his negative answer is correct but even from his combinatorialist viewpoint the rationale he provides for this answer is misguided. I explain why the combinatorialist advocate of concrete plenitude should hold that no object is a tree of possible worlds. Then I explain that for a different reason the nomic essentialist advocate of concrete plenitude should hold this much too. (shrink)

At a first glance, and even at a second one, it seems that if time is linear the threat of determinism is more severe than if time is branching, since in the latter case the future is open in a way it is not in the former one where, so to speak, there exists only one branch – one future. In this paper, I want to give a 'third glance' at this claim. I acknowledge that such a claim is intuitive (...) (this is the first glance) and that it is also meaningfully and interestingly defended in recent literature where branching time is either said to imply indeterminism or at least to be compatible with it. To try to make my third glance as precise and as fleshed out as possible, I shall first concentrate on what 'branching' is or could be, and I shall discuss various versions and interpretations of this view. I shall then (more quickly) turn my attention to what determinism is or could be, and I will distinguish three (well-known) kinds of it – focusing mainly on 'metaphysical determinism'. Having these tools in hand, I will then ask (and answer) the question whether branching time helps with avoiding determinism or not. As we shall see, it is incorrect to think that under the branching hypothesis the threat of determinism is any smaller – rather, I will argue that if one has reasons to think that determinism is true, branching will not help, and that the issue of branching versus linear time is then actually neutral with respect to the question whether determinism or indeterminism is true. (shrink)

The prospect, in terms of subjective expectations, of immortality under the no-collapse interpretation of quantum mechanics is certain, as pointed out by several authors, both physicists and, more recently, philosophers. The argument, known as quantum suicide, or quantum immortality, has received some critical discussion, but there hasn't been any questioning of David Lewis's point that there is a terrifying corollary to the argument, namely, that we should expect to live forever in a crippled, more and more damaged state, that barely (...) sustains life. This is the prospect of eternal quantum torment. Based on some empirical facts, I argue for a conclusion that is much more reassuring than Lewis's terrible scenario.1. (shrink)

This paper examines two mistakes regarding David Lewis’ Principal Principle that have appeared in the recent literature. These particular mistakes are worth looking at for several reasons: The thoughts that lead to these mistakes are natural ones, the principles that result from these mistakes are untenable, and these mistakes have led to significant misconceptions regarding the role of admissibility and time. After correcting these mistakes, the paper discusses the correct roles of time and admissibility. With these results in hand, the (...) paper concludes by showing that one way of formulating the chance–credence relation has a distinct advantage over its rivals. (shrink)

Difficulties over probability have often been considered fatal to the Everett interpretation of quantum mechanics. Here I argue that the Everettian can have everything she needs from `probability' without recourse to indeterminism, ignorance, primitive identity over time or subjective uncertainty: all she needs is a particular *rationality principle*. The decision-theoretic approach recently developed by Deutsch and Wallace claims to provide just such a principle. But, according to Wallace, decision theory is itself applicable only if the correct attitude to a future (...) Everettian measurement outcome is subjective uncertainty. I argue that subjective uncertainty is not to be had, but I offer an alternative interpretation that enables the Everettian to live without uncertainty: we can justify Everettian decision theory on the basis that an Everettian should *care about* all her future branches. The probabilities appearing in the decision-theoretic representation theorem can then be interpreted as the degrees to which the rational agent cares about each future branch. This reinterpretation, however, reduces the intuitive plausibility of one of the Deutsch -Wallace axioms. (shrink)

Following Lewis, it is widely held that branching worlds differ in important ways from diverging worlds. There is, however, a simple and natural semantics under which ordinary sentences uttered in branching worlds have much the same truth values as they conventionally have in diverging worlds. Under this semantics, whether branching or diverging, speakers cannot say in advance which branch or world is theirs. They are uncertain as to the outcome. This same semantics ensures the truth of utterances typically made about (...) quantum mechanical contingencies, including statements of uncertainty, if the Everett interpretation of quantum mechanics is true. The ‘incoherence problem’ of the Everett interpretation, that it can give no meaning to the notion of uncertainty, is thereby solved. IntroductionMetaphysicsPersonal fissionBranching worldsPhysicsObjections. (shrink)

By way of an example, Lewis imagines your being invited to join Schrödinger’s cat in its box for an hour. This box will either fill up with deadly poison fumes or not, depending on whether or not some radioactive atom decays, the probability of decay within an hour being 50%. The invitation is accompanied with some further incentive to comply (Lewis sets it up so there is a significant chance of some pretty bad but not life-threatening punishment if you don’t (...) get in the box). Lewis argues that the many minds theory implies that you should get in the box with the cat, despite this making it 50% likely you will die. (shrink)

I argue that the theory of chance proposed by David Lewis has three problems: (i) it is time asymmetric in a manner incompatible with some of the chance theories of physics, (ii) it is incompatible with statistical mechanical chances, and (iii) the content of Lewis's Principal Principle depends on how admissibility is cashed out, but there is no agreement as to what admissible evidence should be. I proposes two modifications of Lewis's theory which resolve these difficulties. I conclude by tentatively (...) proposing a third modification of Lewis's theory, one which explains many of the common features shared by the chance theories of physics. (shrink)

In this paper I critically examine an argument proposed by Graham Priest in support of the claim that the observable world is consistent. According to this argument we have good reason to think that the observable world is consistent, specifically we perceive it to be consistent. I critique this argument on two fronts. First, Priest appears to reason from the claim 'we know what it is to have a contradictory perception' to the claim 'we know what it is to perceive (...) a contradiction'. I argue that this inference fails to be valid. Secondly, I give reasons for thinking that if an observable state of affairs were to be contradictory, we would perceive it to be consistent. As such that the world we observe appears consistent does not constitute evidence that it is in fact consistent. That we see a consistent world is no reason to believe that the world is consistent. I conclude the paper with some reflections on the implications of this analysis for the plausibility of trivialism. (shrink)

The experimental setup of the self-referential quantum measurement, jovially known as the ‘quantum suicide’ or the ‘quantum Russian roulette’ is analyzed from the point of view of the Principal Principle of David Lewis. It is shown that the apparent violation of this principle – relating objective probabilities and subjective chance – in this type of thought experiment is just an illusion due to the usage of some terms and concepts ill-defined in the quantum context. We conclude that even in the (...) case that Everett’s (or some other ‘no-collapse’) theory is a correct description of reality, we can coherently believe in equating subjective credence with objective chance in quantum-mechanical experiments. This is in agreement with results of the research on personal identity in the quantum context by Parfit and Tappenden. (shrink)

The Everett (many-worlds) interpretation of quantum mechanics faces a prima facie problem concerning quantum probabilities. Research in this area has been fast-paced over the last few years, following a controversial suggestion by David Deutsch that decision theory can solve the problem. This article provides a non-technical introduction to the decision-theoretic program, and a sketch of the current state of the debate.

در نگاه نخست به نظر می‌رسد می‌توان شباهت‌هایی بین جهان‌های ممکن مورد نظر لوئیس و نظریۀ جهان‌های موازی فیزیک یافت. هر دو این نظریات به امکان وجود جهان‌هایی بیش از جهانی که در آن زندگی می‌کنیم، اشاره دارند. پس از رسالۀ اورت، توجه فیزیک‌دانان به چندجهانی و جهان‌های موازی جلب شد. اکنون جهان‌های موازی یکی از نظریه‌های مورد توجه دانشمندان و پژوهشگران حوزۀ فیزیک است. دیوید لوئیس با تکیه بر فیزیکالیسم و رئالیسم موجهاتی از جمله طرفداران نظریۀ جهان‌های ممکن است. (...) در این مقاله کوشیده‌ایم با استفاده از روش تحلیلی-توصیفی ابتدا مبانی دو نظریه را تبیین کرده و سپس نظریۀ لوئیس را در مورد جهان‌های ممکن با توجه به نظریۀ جهان‌های موازی مورد بررسی قرار دهیم. هر دو نظریه وجود جهان‌های دیگر را ممکن می‌دانند. نظریه لوئیس شش ویژگی اصلی دارد: ۱- وجود و واقعیت جهان‌های ممکن، ۲- عدم وجود رابطه علی میان‌ جهان‌های ممکن، ۳- شباهت جهان‌های ممکن با جهان ما و تفاوت در محتوای جهان‌ها، ۴- عدم امکان تقلیل جهان‌های ممکن، ۵- نمایه‌ای بودن واقعیت و ۶- اتحاد مکان- زمان اجزای هر جهان و مجزا بودن روابط زمان-مکان جهان‌ها. نتایج پژوهش نشان می‌دهد در صورتی که جهان‌های موازی، زمانی به عنوان یک حقیقت علمی مورد تأیید قرار بگیرند، آنگاه ممکن است ویژگی‌های دوم، سوم و ششم نظریۀ لوئیس با واقعیت فیزیکی در تضاد قرار بگیرند. در صورت وجود تضاد -به جهت بنیادی بودن این ویژگی‌ها- نظریۀ جهان‌های ممکن لوئیس دچار تزلزل فراوان خواهد شد. (shrink)

In this review, I specify the metametaphysical background against which Alastair Wilson’s “_The Nature of Contingency_” (Oxford University Press, 2020) should be properly understood. Metaphysics, as a philosophical discipline, is standing on thin ice. The caricature of the situation is polarized, and is often presented as follows: metaphysics is either entirely extracted from science or it is entirely independent of science. There is a recent trend that focuses on the middle ground between these extremes, searching the philosophical literature for metaphysical (...) theories that can fill the gap, i.e., leaving metaphysics as a free discipline to produce spoils for the eventual needs of philosophers of science. We can appreciate it better with the following distinction between the tasks of ontology and metaphysics, as complementary disciplines. If, on the one hand, we understand ontology as dealing with what exists, we can somehow extract the entities that are existentially postulated by scientific theories. Metaphysics, on the other hand, would be located as an extra layer, in charge of investigating questions about the nature of the entities obtained in this “naturalized ontology”. As a tailor, Wilson adjusts a metaphysical theory in order to perfectly dress the physical and ontological nuances of Everettian quantum mechanics, thus creating a metaphysical theory that gives us intelligibility, with the concept of modality, in two areas: in quantum mechanics, and analytic metaphysics. (shrink)

Nonrelativistic quantum mechanics (QM) works perfectly well for all practical purposes. Once one admits, however, that a successful scientific theory is supposed not only to make predictions but also to tell us a story about the world in which we live, a philosophical problem emerges: in the specific case of QM, it is not possible to associate with the theory a unique scientific image of the world; there are several images. The fact that the theory may be compatible with distinct (...) ontologies, and that those ontologies may themselves be associated with a plurality of metaphysical approaches, gives rise to the problem of metaphysical underdetermination. This paper concludes that the available metametaphysical criteria fail to deliver objectivity in theory choice, and it puts forward its own criterion based on a tension between two methods of metaphysical inquiry: one that is closely related to science and another that is not. (shrink)

We ponder the question whether quantum physics has had any influence on philosophy, and if not, whether it ought to have had any. Answers to these questions are provided, and they turn out to depend onwhichbranch of the tree of philosophy we sweep, sway and swing, and evenwhichtwig of the branch we touch when we sweep, sway and swing.

This article explores theoretical conditions necessary for “quantum immortality” as well as its possible practical implications. It is demonstrated that QI is a particular case of “multiverse immortality”, which is based on two main assumptions: the very large size of the universe ; and a copy-friendly theory of personal identity. It is shown that a popular objection about lowering of the world-share of an observer in the case of QI does not succeed, as the world-share decline could be compensated by (...) merging timelines for the simpler minds, and because some types of personal preferences are not dependent on such changes. Despite large uncertainty about the truth of MI, it has appreciable practical consequences for some important outcomes like suicide and aging. The article demonstrates that MI could be used to significantly increase the expected subjective probability of success of risky life extension technologies, such as cryonics, but that it makes euthanasia impractical because of the risk of eternal suffering. Euthanasia should be replaced with cryothanasia, i.e. cryopreservation after voluntary death. Another possible application of MI is as a last chance to survive a global catastrophe. MI could be considered a Plan D for reaching immortality, where Plan A consists of survival until the development of beneficial Artificial Intelligence capable of fighting aging, Plan B employs cryonics, and Plan C is digital immortality. (shrink)

Humean Supervenience is a metaphysical model of the world according to which all truths hold in virtue of nothing but the total spatiotemporal distribution of perfectly natural, intrinsic properties. David Lewis and others have worked out many aspects of HS in great detail. A larger motivational question, however, remains unanswered: As Lewis admits, there is strong evidence from fundamental physics that HS is false. What then is the purpose of defending HS? In this paper, we argue that the philosophical merit (...) of HS is largely independent of whether it correctly represents the world’s fundamental structure. In particular, we show that insofar as HS is an apt model of the world’s higher-level structure, it thereby provides a powerful argument for reductive physicalism and explains otherwise opaque inferential relations. Recent criticism of HS on the grounds that it misrepresents fundamental physical reality is, therefore, beside the point. (shrink)

The possibility of algorithmic consciousness depends on the assumption that conscious states can be copied or repeated by sufficiently duplicating their underlying physical states, leading to a variety of paradoxes, including the problems of duplication, teleportation, simulation, self-location, the Boltzmann brain, and Wigner’s Friend. In an effort to further elucidate the physical nature of consciousness, I challenge these assumptions by analyzing the implications of special relativity on evolutions of identical copies of a mental state, particularly the divergence of these evolutions (...) due to quantum fluctuations. By assuming the supervenience of a conscious state on some sufficient underlying physical state, I show that the existence of two or more instances, whether spacelike or timelike, of the same conscious state leads to a logical contradiction, ultimately refuting the assumption that a conscious state can be physically reset to an earlier state or duplicated by any physical means. Several explanatory hypotheses and implications are addressed, particularly the relationships between consciousness, locality, physical irreversibility, and quantum no-cloning. (shrink)

Plausible assumptions from Cosmology and Statistical Mechanics entail that it is overwhelmingly likely that there will be exact duplicates of us in the distant future long after our deaths. Call such persons “Boltzmann duplicates,” after the great pioneer of Statistical Mechanics. In this paper, I argue that if survival of death is possible at all, then we almost surely will survive our deaths because there almost surely will be Boltzmann duplicates of us in the distant future that stand in appropriate (...) relations to us to guarantee our survival. (shrink)

Some interpretations of quantum mechanics use notions of possible states and possible trajectories. We investigate how this modal approach correlates with several metaphysical conceptions of a transition from potential to actual existence. The comparison is based on a discussion in contemporary analytic metaphysics of modality. We also consider an analogy between Leibniz’s theory of the possibilities striving towards existence, wave mechanics of Schrödinger, and Feynman path integral.

A number of cases involving self-locating beliefs have been discussed in the Bayesian literature. I suggest that many of these cases, such as the sleeping beauty case, are entangled with issues that are independent of self-locating beliefs per se. In light of this, I propose a division of labor: we should address each of these issues separately before we try to provide a comprehensive account of belief updating. By way of example, I sketch some ways of extending Bayesianism in order (...) to accommodate these issues. Then, putting these other issues aside, I sketch some ways of extending Bayesianism in order to accommodate self-locating beliefs. Finally, I propose a constraint on updating rules, the "Learning Principle", which rules out certain kinds of troubling belief changes, and I use this principle to assess some of the available options. (shrink)

It is shown that Lewis’ ontological doctrine of Humean supervenience incorporates at its foundation the so-called separability principle of classical physics. In view of the systematic violation of the latter within quantum mechanics, the claim that contemporary physical science may posit non-supervenient relations beyond the spatiotemporal ones is reinforced on a foundational basis concerning constraints on the state representation of physical systems. Depending on the mode of assignment of states to quantum systems — unit state vectors versus statistical density operators (...) — we distinguish between strongly and weakly non-Humean, non-supervenient relations. It is demonstrated that in either case, the relations of quantum entanglement constitute prototypical examples of irreducible physical relations that do not supervene upon a spatiotemporal arrangement of Humean qualities, weakening, thereby, the thesis of Humean supervenience. In this respect, the status of Lewis’ recombination principle is examined, whereas his conception of lawhood is critically investigated. It is concluded that the assumption of ontological reductionism, as expressed in Lewis’ Humean doctrine, cannot be regarded as a reliable code of the nature of the physical world and its contents. It is proposed instead that due to the undeniable existence of non-supervenient relations, a metaphysic of relations of a moderate kind ought to be acknowledged as an indispensable part of our understanding of the natural world at a fundamental level. (shrink)

How does it come about then, that great scientists such as Einstein, Schrödinger and De Broglie are nevertheless dissatisfied with the situation? Of course, all these objections are levelled not against the correctness of the formulae, but against their interpretation. [...] The lesson to be learned from what I have told of the origin of quantum mechanics is that probable refinements of mathematical methods will not suffice to produce a satisfactory theory, but that somewhere in our doctrine is hidden a (...) concept, unjustified by experience, which we must eliminate to open up the road. (Born [ 1954 ], pp. 8, 11) It is truly surprising how little difference all this makes. Most physicists use quantum mechanics every day in their working lives without needing to worry about the fundamental problem of its interpretation. (Weinberg [ 1992 ], p. 66) I endorse the view that it may be of no relevance to the acceptability of the Everett interpretation of quantum mechanics as a physical theory whether or not an informed observer can be uncertain about the outcome of a quantum measurement prior to its having occurred. However, I suggest that the very possibility of post-measurement, pre-observation uncertainty has an essential role to play in both confirmation theory and decision theory in a branching universe. This is supported by arguments which do not appeal to van Fraassen’s Reflection Principle. (shrink)

David Lewis’ contemplations regarding divine foreknowledge and free will, along with some of his other more substantial work on modal realism and his counterpart theory can serve as a springboard to a novel solution to the foreknowledge and metaphysical freedom puzzle, namely a proposal that genuine metaphysical freedom is compatible with determinism, which is quite different from the usual compatibilist focus on the compatibility between determinism and moral responsibility. This paper argues that while Lewis opens the doors to such a (...) possibility, in order to fully elucidate a genuinely metaphysical compatibilist account, Lewis’ own counterpart theory must be abandoned in favour of an account of trans-world identity that is theoretically framed by a modified version of Robert Nozick’s closest continuer theory. (shrink)

Personal identity is not always symmetric: even if I will not be a later person, the later person may have been me. What makes this possible is that the relations that are criterial of personal identity---such as memory and anticipation---are asymmetric and "count in favor of personal identity from one side only". Asymmetric personal identity can be accommodated by temporal counterpart theory but not by Lewisian overlapping aggregates of person stages. The question of uncertainty in cases of personal fission (and (...) in Everettian quantum mechanics) is also discussed. (shrink)

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)

The Everett interpretation of quantum mechanics divides naturally into two parts: first, the interpretation of the structure of the quantum state, in terms of branching, and second, the interpretation of this branching structure in terms of probability. This is the second of two reviews of the Everett interpretation, and focuses on probability. Branching processes are identified as chance processes, and the squares of branch amplitudes are chances. Since branching is emergent, physical probability is emergent as well.

Possibilism—the view that there are non-actual, merely possible entities—is a surprisingly resilient doctrine.1 One particularly hardy strand of possibilism—the modal realism championed by David Lewis—continues to attract both foes who seek to demonstrate its falsity (or at least stare its advocates into apostasy) and friends who hope to defend modal realism (or, when necessary, modify modal realism so as to avoid problematic objections).2 Although I am neither a foe nor friend of modal realism (but some of my best friends are!), (...) like many I continue to be fascinated by the doctrine. (shrink)

David Lewis’s attempt to defuse grandfather paradoxes consistently without special restrictions on the ability of time travelers to act in the past is controversial. Kadri Vihvelin uses the case of possible autoinfanticide—killing one’s infant self—to argue on Lewisian grounds that Lewis is wrong, since all counterfactual attempts at autoinfanticide would fail. I present a new defense of Lewis against Vihvelin premised on the possibility of personal _reinstatement_, where a person who dies prematurely is replicated from information collected from a previous (...) live scan. I argue on Lewisian grounds that in a Vihvelin case where Suzy does not attempt to kill Baby Suzy, Vihvelin has not shown that Suzy would have failed had she tried to kill Baby Suzy. For, Baby Suzy might have been reinstated. Hence, even granting Vihvelin’s own assumptions, a Lewisian can assert that Suzy can kill Baby Suzy. Reinstatement does not require a “big” miracle; so autoinfanticide is no biggie. (shrink)

David Lewis cautions that although a no-collapse interpretation of quantum mechanics entails immortality for trans-world selves, the nature of the branching leaves us crippled, lonely, deathly ill (although never dead), and mentally infirm, meaning that immortal life, on such terms, amounts to an existence in eternal torment. This paper argues that the problem Lewis points to is in fact one of individuation and that a synthesis of Lewis’ own notion of perdurance and Robert Nozick’s closest continuer theory, when cast in (...) the mould of a deterministic multiverse (as conceived by the Oxford quantum physicist David Deutsch), individuates trans-world selves in such a way as to allow to prune the infinitesimal expectation of a miserable eternal existence from the histories of most trans-world agents. Thus, contrary to Lewis’ warning that if personal identity is a trans-world notion, then we should all shake in our shoes, this paper argues that even if we are trans-world selves, we almost certainly have nothing to worry about. (shrink)

A novel puzzle for the notion of probability in the Many-Worlds interpretation of quantum mechanics is presented. The puzzle makes use of a thought experiment that some have claimed would provide empirical support for Many-Worlds over alternatives. It is argued that, if the predictions of Many-Worlds do indeed differ from other interpretations as claimed, then Born’s rule must generally be invalid in Many-Worlds. It is shown that the thought experiment provides a counter example for recent decision-theoretic arguments that purport to (...) establish Born’s rule. Finally, it is shown that the puzzle can be grounded in general considerations regarding the nature of prediction in Many-Worlds. (shrink)

We discuss the meaning of probabilities in the many worlds interpretation of quantum mechanics. We start by presenting very briefly the many worlds theory, how the problem of probability arises, and some unsuccessful attempts to solve it in the past. Then we criticize a recent attempt by Deutsch to derive the quantum mechanical probabilities from the nonprobabilistic parts of quantum mechanics and classical decision theory. We further argue that the Born probability does not make sense even as an additional probability (...) rule in the many worlds theory. Our conclusion is that the many worlds theory fails to account for the probabilistic statements of standard (collapse) quantum mechanics. (shrink)

Magnetism in meta-semantics is the view that the meaning of our words is determined in part by their use and in part by the objective naturalness of candidate meanings. This hypothesis is commonly attributed to David Lewis, and has been put to philosophical work by Brian Weatherson, Ted Sider and others. I argue that there is no evidence that Lewis ever endorsed the view, and that his actual account of language reveals good reasons against it.

In this dissertation, I defend a Russellian form of descriptivism. The main supporting argument invokes a relation between meaning and thought. I argue that the meanings of sentences are the thoughts people use them to express. This is part of a Gricean outlook on meaning according to which psychological intentionality is prior to, and determinative of, linguistic intentionality. The right approach to thought, I argue in Chapter 1, is a type of functionalism on which thoughts have narrow contents. On this (...) view, the attitude ascriptions of a regimented psychology capture what people really believe and desire. These attitude ascriptions have content clauses that are what David Lewis calls ‘modified Ramsey sentences.’ I then conclude that, since the meanings of sentences are the narrow contents of the thoughts speakers use them to express, the meanings of sentences can also be represented with such descriptive sentences. I extend the view so that it applies to individual words. The resulting view is a form of descriptivism. Referring, I claim in Chapter 2, is the expression of a de re attitude. I argue that the non-psychological, de re individuation of thoughts captures only contingent features of these thoughts. Furthermore, whether a thought counts as de re depends on the attributor’s context. These two characteristics carry over to reference. The referential properties of speech acts and expressions are merely contingent features. Furthermore, whether a speech act or expression counts as referring depends upon the attributor’s context. In Chapter 3, I apply this version of descriptivism to indexicals, demonstratives and names. Indexicals turn out to have non-descriptive, context-insensitive, semantically determined meanings. Demonstratives have descriptive, context-sensitive, pragmatically determined meanings. Names, finally, have descriptive, context-insensitive, semantically determined meanings. In the final chapter, I address Putnam's model-theoretic argument, the most formidable obstacle to the form descriptivism outlined here. I criticize Lewis's ‘magnetist’ solution that invokes primitive naturalness because it is committed to the existence of incorrigible error about the external world. I suggest an empiricist approach on which psychological intentionality, and so ultimately linguistic intentionality as well, is anchored in experience. (shrink)

Arguments from fission cases, most notably made by Parfit, have historically been utilized in discussions of Everettian quantum mechanics (EQM) in an attempt to illuminate details of familiar accounts in which an agent ‘splits’. Whilst such imagery is often seen as an innocuous depiction of Everett's theory, it is in fact a poisoned chalice. I argue firstly that the fission case analogy is responsible for the conceptual foundations of probability arguments in EQM and secondly, following a number of disanalogies between (...) fission cases and Everettian branching, I argue that the analogy is unfounded. I conclude that much of the discussion of the probability problem over the past 20 years has been predicated on a problematic analogy and its effects are still apparent today. (shrink)