Recent work on probability in the Everett interpretation of quantum mechanics yields a decision-theoretic derivation of David Lewis’ Principal Principle, and hence a general metaphysical theory of probability; part 1 is a discussion of this remarkable result. I defend the claim that the ‘subjective uncertainty’ principle is required for the derivation to succeed, arguing that it amounts to a theoretical identification of chance. In part 2, I generalize this account, and suggest that the Everett interpretation, in combination with a plausible view of natural laws, has the potential to provide a reductive theory of metaphysical modality. I defend the resulting naturalistic modal realism, and outline some of its implications for other parts of metaphysics.
Recent work on probability in the Everett interpretation of quantum mechanics yields a decision-theoretic derivation of David Lewis’ Principal Principle, and hence a general metaphysical theory of probability; part 1 is a discussion of this remarkable result. I defend the claim that the ‘subjective uncertainty’ principle is required for the derivation to succeed, arguing that it amounts to a theoretical identification of chance. In part 2, I generalize this account, and suggest that the Everett interpretation, in combination with a plausible view of natural laws, has the potential to provide a reductive theory of metaphysical modality. I defend the resulting naturalistic modal realism, and outline some of its implications for other parts of metaphysics.
This article sits at a point of intersection between the philosophy of physics and the metaphysics of modality. There are clear similarities between Everettian quantum mechanics and various modal metaphysical theories, but there have hitherto been few attempts at exploring how the two topics relate. In this article, I build on a series of recent papers by Wilson ([2011], [2012], [2013]), who argues that Everettian quantum mechanics’ connections with traditional modal metaphysics are vital in defending it against objections. I show that Wilson’s preferred version of Everettian quantum mechanics has two problems. First, it is unable to account for the contingency of various intuitively contingent modal claims. Second, it fails to yield intuitive truth values on modal claims about the number of branches in a given Everettian multiverse. Since modal claims about branch number are instrumental in decision-theoretic solutions to Everettian quantum mechanics’ problem(s) with probability, this second problem has wider dialectical implications. I suggest amendments to the underlying metaphysics that overcome these problems. The result is a more robust version of Everettian quantum mechanics.
Contingency is everywhere, but what is it? This book defends a radical new theory of contingency as a physical phenomenon. Drawing on the many-worlds approach to quantum theory and on 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. The framework is a modal realist one, in the tradition of David Lewis: all genuine possibilities are on a par, and the actual world is simply the one that we ourselves inhabit. It departs from Lewisian modal realism in that quantum possible worlds are not philosophical posits but scientific discoveries. Contingency and other modal notions have often been seen as beyond the limits of science. 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.
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. 1. Introduction 2. What is probability? 2.1Objective probability and the Principal Principle 2.2Three ways of satisfying the functional definition 2.3Cautious functionalism 2.4Is the functional definition complete? 3. The Everett interpretation and subjective uncertainty 3.1Interpreting quantum mechanics 3.2The need for subjective uncertainty 3.3Saunders' argument for subjective uncertainty 3.4Objections to Saunders' argument 3.5Subjective uncertainty again: arguments from interpretative charity 3.6Quantum weights and the functional definition of probability 4. Rejecting subjective uncertainty 4.1The fission program 4.2Against the fission program 5. Justifying the axioms of decision theory 5.1The primitive status of the decision-theoretic axioms 5.2Holistic scepticism 5.3The role of an explanation of decision theory 6. Conclusion
Abstract Contingency is everywhere, but what is it? This book defends a radical new theory of contingency as a physical phenomenon. Drawing on the many-worlds approach to quantum theory and on 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. The framework is a modal realist one, in the tradition of David Lewis: all genuine possibilities are on a par, and the actual world is simply the one that we ourselves inhabit. It departs from Lewisian modal realism in that quantum possible worlds are not philosophical posits but scientific discoveries. Contingency and other modal notions have often been seen as beyond the limits of science. 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.
David Lewis' "Principal Principle" is a purported principle of rationality connecting credence and objective chance. Almost all of the discussion of the Principal Principle in the philosophical literature assumes classical probability theory, which is unfortunate since the theory of modern physics that, arguably, speaks most clearly of objective chance is the quantum theory, and quantum probabilities are not classical probabilities. This paper develops an account of how chance works in quantum theory that reveals a connection between credence and quantum chance quite unlike what is envisioned in the philosophical literature: as a theorem of quantum probability, updating a completely additive chance function on a knowledge of chance brings credence into line with chance. The account also suggests a way of construing the Humean supervenience of chance that has the virtue of dissolving some puzzles about the "undermining" of chances. A number of interpretative moves in quantum theory are needed to generate the account of quantum chance on offer here, and they can all be disputed. But engaging in these disputes is part and parcel of naturalized metaphysics, and as such it can be more productive than engaging in the battle of intuitions among analytical metaphysicians about how chance ought to work this and other possible worlds.
Abstract This chapter offers a theory of objective chance in the Everettian context, often seen as the main challenge facing EQM. By supplementing diverging EQM with quantum modal realist bridge principles connecting the physics of quantum mechanics with the metaphysics of modality, we obtain a package deal: Indexicalism. Indexicalist objective chance is an essentially self-locating phenomenon: chances are chances of self-location within the multiverse. I provide three arguments for Indexicalism: it establishes the right qualitative connections between chance and possibility, it establishes the right quantitative connection between chance and prediction, and it establishes the right epistemological story about how quantum mechanics is confirmed by empirical evidence. The resulting theory of chance is naturalistic and reductive; fundamental reality is deterministic, but chance arises at the non-fundamental level of Everett-worldbound perspectives. The theory provides unique resources for motivating an Everettian version of Lewis’s Principal Principle, helping to clarify at last the persistently mysterious connection between chance and rational credence.