Ontology for Collapse Theories

In this chapter, I will discuss what it takes for a dynamical collapse theory to provide a reasonable description of the actual world. I will start with discussions of what is required, in general, of the ontology of a physical theory, and then apply it to the quantum case. One issue of interest is whether a collapse theory can be a quantum state monist theory, adding nothing to the quantum state and changing only its dynamics. Although this was one of the motivations for advancing such theories, its viability has been questioned, and it has been argued that, in order to provide an account of the world, a collapse theory must supplement the quantum state with additional ontology, making such theories more like hidden-variables theories than would first appear. I will make a case for quantum state monism as an adequate ontology, and, indeed, the only sensible ontology for collapse theories. This will involve taking dynamical variables to possess, not sharp values, as in classical physics, but distributions of values