Chance Debugged

A ‘Big Bad Bug’ threatens Lewis’s Humean metaphysics of chance (Lewis 1986a, p. XIV); his Principal Principle provides an intuitive link between chance and credence. Yet on the one hand, certain future developments are incompatible with the true theory of chance, but on the other hand, such future developments have a positive chance to occur. The combination of these two claims with the Principal Principle leads to inconsistent credences. I present a Humean solution to the Bug: chances are relative to a limited perspective. The perspective comprises facts available as evidence to an ideal cognizer at a point in space-time. As a consequence, the same future event can have different chances of occurring provided the perspective is different. I show how this dissolves the Bug.

The Principal Principle, admissibility, and normal informal standards of what is reasonable

This paper highlights the role of Lewis’ Principal Principle and certain auxiliary conditions on admissibility as serving to explicate normal informal standards of what is reasonable. These considerations motivate the presuppositions of the argument that the Principal Principle implies the Principle of Indifference, put forward by Hawthorne et al. (British Journal for the Philosophy of Science, 68, 123–131, 2017). They also suggest a line of response to recent criticisms of that argument, due to Pettigrew (British Journal for the Philosophy of Science, 71, 605–619, 2020) and Titelbaum and Hart (British Journal for the Philosophy of Science, 71(2), 621–632, 2020). The paper also shows that related concerns of Hart and Titelbaum (Thought: A Journal of Philosophy, 4(4), 252–262, 2015) do not undermine the argument of Hawthorne et al. (2017).

Objective Bayesianism and Knowledge-First Epistemology

This chapter distinguishes between subjective and objective versions of Bayesianism, arguing for the latter variety. The distinction is made in terms of constraints on ur-priors. Lewis’s “Principal Principle” is discussed as one such constraint. The Carnapian program of delineating a unique ur-prior in purely syntactical terms is presented, and rejected for familiar reasons. It is then argued that the failure of the Carnapian program does not entail the failure of Objective Bayesianism more generally. Ur-prior Conditionalization is introduced as a better alternative to Conditionalization. The chapter ends by presenting Factualism, the view that our evidence consists of what we know.

Emergent Chance

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.