 We will provide a more detailed analysis of these examples below, but we can already see that a family of—sometimes competing—notions is beginning to emerge. However, we should formulate each notion somewhat more precisely. The first thing to note in defining ontological dependence is the modal-existential element in dependence claims. For instance, we’ve said that a set cannot exist unless its members do. So there is a sense in which the existence of a set necessitates the existence of its members. Indeed, it is common to talk about, e.g., rigid existential necessitation as synonymous with rigid existential dependence. Typically, statements of ontological dependence are thought to refer to metaphysical modality (rather than, say, conceptual or logical modality), primarily because they concern matters that are broader than just conceptual or logical. For instance, substances are often considered to be entities that do not depend, in a distinctively metaphysical sense, for their existence upon anything else (see Lowe 1998: Ch. 6). We can see this already in Descartes, who asserts that