 The sort of properties that have traditionally been taken to support the classificatory practices relevant to TP5 are intrinsic to the individual organisms in question. Moreover, they have been taken to be able to fulfil this role in virtue of being necessary and sufficient for the organism’s membership of the species, i.e., “essential” in one meaning of the term. This view of species membership, and the associated view of species themselves, has been influentially dubbed “typological thinking” (Mayr 1959 [1976: 27f.]; cf. Mayr 1982: 260) and “essentialism” (Hull 1965: 314ff.; cf. Mayr 1968 [1976: 428f.]). The former characterisation involves an epistemological focus on the classificatory procedure, the latter a metaphysical focus on the properties thus singled out. Ernst Mayr claimed that the classificatory approach originates in Plato’s theory of forms, and, as a result, involves the further assumption that the properties are unchanging. According to David Hull, its root cause is the attempt to fit the ontology of species taxa to an Aristotelian theory of definition.