Mathematics can provide precise formulations of relatively vague concepts and problems from the real world, and bring out underlying structure common to diverse scientific areas. Sometimes very natural mathematical concepts lie neglected and not widely understood for many years, before their fundamental relevance is recognised and their explanatory power is fully exploited. The notion of definability in a structure is such a concept, and Turing's [77] 1939 model of interactive computation provides a fruitful context in which to exercise the usefulness of definability as a powerful and widely applicable source of understanding. In this article we set out to relate this simple idea to one of the oldest and apparently least scientifically approachable of problems -that of realistically modelling how mental properties supervene on physical ones.Mathematics can provide precise formulations of relatively vague concepts and problems from the real world, and bring out underlying structure common to diverse scientific areas. Sometimes very natural mathematical concepts lie neglected and not widely understood for many years, before their fundamental relevance is recognised and their explanatory power is fully exploited. Previously we have argued that the notion of definability in a structure is such a concept, and pointed to Turing's [77] 1939 model of interactive computation as providing a fruitful context in which to exercise the usefulness of definability as a powerful and widely applicable source of understanding.Below, we relate this simple idea to one of the oldest and apparently least scientifically approachable of problems -that of realistically modelling how mental properties supervene on physical ones. We will first briefly review the origins with René Descartes of mind-body dualism, and the problem of mental causation. We will then summarise the subsequent difficulties encountered, and their current persistence, and the more recent usefulness of the concept of supervenience in * Research supported by EPSRC research grant No. Computing with Partial Information: Definability in the Local Structure of the Enumeration Degrees.1 From Descartes to Turing: The Computational Content of Supervenience 2 providing a philosophical workspace in which to make mind-body connectionsand the parallel recognition of emergence as a tool for giving supervenience a non-reductive physical content. The rise and fall of the British emergentists will provide a salutary warning of the pitfalls of working with too vague a formulation of emergence, and of the need for a test for emergence.Following on from this, we will further clarify emergence by looking at mathematical analogues, and at approaches to emergence -such as synergetics -with a strong mathematical or scientific content. This will move us away from the empirical quest for emergent phenomena as something surprising towards formalisations of self-organisation in terms of the two-way interaction between physical phenomena, and their descriptions and representations. We w...