'Constituency' depends on some layering of the world described by the ontology. For example, scientific granularities (e.g. body-organ-tissue-cell) or ontological 'strata' (e.g. social-mental-biological-physical) are typical layerings. Intuitively, a constituent is a part belonging to a lower layer. Since layering is actually a partition of the world described by the ontology, constituents are not properly classified as parts, although this kinship can be intuitive for common sense. A desirable advantage of this distinction is that we are able to talk e.g. of physical constituents of non-physical objects (e.g. systems), while this is not possible in terms of parts. Example of are the persons constituting a social system, the molecules constituting a person, the atoms constituting a river, etc. In all these examples, we notice a typical discontinuity between the constituted and the constituent object: e.g. a social system is conceptualized at a different layer from the persons that constitute it, a person is conceptualized at a different layer from the molecules that constitute them, and a river is conceptualized at a different layer from the atoms that constitute it.