Alastair Wilson

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.