Chapter 6 will examine what exactly is presupposed about the metaphysical character of the relation between parts and compounds in the context of part-whole explanations. These kinds of explanations have often been taken to be evidence for Physical Foundationalism, a view that assumes that an ontological priority relation obtains between the micro-level and the macro-level. I will argue that part-whole explanations (just as other explanations) do indeed presuppose the existence of dependence relations between what the explanans refers to and what the explanandum refers to (this is sometimes called a ‘backing relation’). However, the stronger claim that an ontological priority relation obtains in nature does not do any work in understanding the dependence relations involved in our reductive explanatory practices. All we need is the assumption that parts and wholes mutually determine each other. A minimal metaphysics of science needs to postulate a dependence relation but not an ontological priority relation. Foundationalism is not implied by what classical mechanics and quantum mechanics have to say about the part-whole relation.