Full Idea
Justice, as has been rightly said, consists in each one fulfilling his [authentic and proper] function.
Gist of Idea
Justice is each person fulfilling his function
Source
Porphyry (Launching Points to the Realm of the Mind [c.280], 6Enn5 44)
Book Reference
Porphyry: 'Launching Points to the Realm of the Mind', ed/tr. Guthrie,Kenneth [Phanes 1988], p.69
A Reaction
This is presumably a direct reference to the theory in Plato's 'Republic'. It makes the connection between virtue and function which I take to be basic to virtue theory, giving it a naturalistic advantaged over other theories.