Genus and Species

Genus and species are the two halves of a proper definition: the genus is the broad kind a thing belongs to (animal), and the species is the narrower group within it, marked off by a distinguishing trait called the differentia (rational). Aristotle made this pairing the backbone of classification, and the later philosopher Porphyry arranged it into the famous "Porphyrian tree," a ladder descending from the most general category down to particular individuals. The same impulse appears in Plato, who sought definitions by repeatedly dividing a broad kind in two. Together these thinkers gave Western thought its enduring picture of reality as a nested hierarchy of universals.