John Searle (1992).
Ontological reduction: The most important type of reduction, in which an object is shown to consist solely of another type of object. For example, a gene consists solely of DNA molecules.
Causal reduction: a form of reduction in which the causal powers of some object are shown to be entirely the result of the causal powers of another type of object. For example ,the causal powers of a solid object—i.e. to remain impenetrable—are explainable in terms of the causal powers of molecules in its lattice structures.