Wholes are often distinguished from arbitrary mereological sums. Wholes are taken to be very special mereological entities that for some are shrouded in mystery. In applied ontology, however, e.g., in the ontology of the biomedical sciences, it is useful to distinguish certain categories of entities that stand in special relations to their parts. Such categories are collectives and compounds (Schulz et al. 2006), corresponding to the relations of granular parthood and determinate parthood (Rector et al. 2005). Collectives are thought to consist of ‘grains’ that are all of the same kind, which are normally not spatially connected and whose number can vary as long as there is at least one grain left – although some authors allow for collectives with no grain at all (Rector et al. 2005), whereas others require at least two grains to be present (Wood & Galton 2009). It is important to note that the uniformity of grains is relative to a certain level of granularity and specificity – a collection of H2O molecules is uniform at the level of molecules, but not on the level of atoms or subatomic particles, and it could also be considered to be a mixture of oxides of different hydrogen isotopes (like heavy water or super-heavy water). Similarly, a collection of jewels can be considered to be a mono-sortal collection of stones, but also a multisortal mixture of emeralds and diamonds. Compounds, on the other hand, typically consist of a fixed number of components that are spatially connected but may be of different kinds.
1 Jansen, L.; Schulz, St., (2011), “Grains, Components and Mixtures in Biomedical Ontologies”, Journal of Biomedical Semantics 2 (Suppl. 4), S2 (9. August 2011).
2 Rector, A.; Rogers, J.; Bittner, Th., (2005), “Granularity, Scale and Collectivity: When Size Does and Does not Matter”, Journal of Biomedical Informatics 39: 333-349.
3 Schulz, St.; Beisswanger, E.; Hahn, U.; Wermter, J.; Kumar, A.; Stenzhorn, H., (2006), “From GENIA to BioTop. Towards a top-level Ontology for Biology”, in: Proceedings of the International Conference on Formal Ontology in Information Systems (FOIS 2006). Baltimore MD, USA. Amsterdam: IOS:103-114.
4 Wood, Z. M.; Galton, A. P., (2009), “A Taxonomy of Collective Phenomena”, Applied Ontology 4: 267-292.