Some Aristotelian Logical Terminology

and the Tree of Porphyry

Species (pl. species; Gk. eidos, form or idea) — What is predicable of many individuals that are of the same kind; precisely or “specifically” what kind of thing something is.  Hence many individuals of the same species are “specifically” the same.

Genus (pl. genera; Gk. genos, kind or class) — What is predicable of many individuals differing in species (and so also predicable of many species); generally or “generically” what kind of thing something is.  Hence many individuals of the same genus are “generically” the same.

Differentia (pl. differentiae; Gk. diaphora, difference) — What determines a member of a genus to be a member of one species and not another.  Hence a species is defined by the conjunction of its genus and its differentia: man is defined as rational animal (animal is the genus, rational is the difference).

Note: Genus and species are relative terms, so that mammal is a species of the genus animal, but mammal is also the genus of the species man.  Hence:

Species specialissima (“most specific species”) — The most specific or proper species of an individual. (In this sense mammal is not a species, but man is.)

Genus supremum (“highest genus”) — The most general or widest kind to which something belongs.  (In this sense substance is a genus, but animal is not).