Ontological dependence

Ontological dependence is the relation by which one entity exists in virtue of another, or one fact obtains in virtue of another. A set depends ontologically on its members: there cannot be a set without members to compose it. A shadow depends ontologically on the object that casts it: there cannot be a shadow without something to block the light. This relation is central to analytic metaphysics, where it serves as a criterion for distinguishing fundamental entities from derivative ones, and to systems theory, where it describes the hierarchical stratification of reality from particles to organisms to societies.

The concept is distinct from but related to grounding. Grounding is a relation between facts; ontological dependence is a relation between entities. The fact that the temperature is 20°C is grounded in the kinetic energy of molecules; the temperature-reading itself depends ontologically on the thermometer, the observer, and the physical system being measured. The distinction matters because not all dependence is metaphysical — some is epistemic, some is causal, and some is merely conceptual. The task of contemporary metaphysics is to map these varieties of dependence without collapsing them into a single undifferentiated relation.