and disciple and editor of Plotinus. Porphyry also wrote commentaries upon Plato and Aristotle. His most influential work was the Isagoge or Introduction to Aristotle’s Categories, which was responsible for the classic formulation of the problem of universals, as it preoccupied Boethius and subsequent medieval philosophy. At the beginning of the Isagoge Porphyry asks: ‘Do genera and species subsist in themselves, or do they exist only in the mind? If they subsist in themselves, are they corporeal or incorporeal? If they are incorporeal, do they exist in separation from sensible substances or in conjunction with them?’ The tree of Porphyry is the method of classification by dichotomy: per genus et differentiam. Porphyry’s distinction is as a transmitter of other peoples’ work. Even his major editor and commentator, J. Bidez, in his Vie de Porphyre (1913), confesses that in the entire extant work of Porphyry there ‘is not a thought or an image which one can confidently affirm to be his own’.