Abstract

Most attempts at defining or elucidating ’weak’ or ’strong’ supervenience introduce various forms of _physical indiscernibility_. After glancing at some definitions, I argue that they must fail if mental events are supposed to be genuinely causally efficacious and non-epiphenomenal. Then I elucidate Davidson’s account of supervenience (’D-supervenience’), first as an abstract relation between a predicate and a set of predicates (to be illustrated by uncontroversial examples), and then as applied to the mental/physical relation. I argue that Davidson must defend that if two events are physically indiscernible in the light of a complete physical theory of the world then they must be identical in the Leibnizian sense (‘L-identity’): physical indiscernibility collapses into L-identity. It follows that mental differences between two numerically different events logically entail physical differences between them. I conclude by demonstrating how the Davidsonian account of the concept can explain some counterexamples to Moore’s approach to supervenience of moral on natural properties. The central tenet in this paper is that supervenience principles based on duplication create, rather than solve, problems in this problem area