Avicenna against Porphyry’s Definition of Differentia Specifica

The paper deals with Avicenna’s polemical attitude towards the traditional definition of differentia specifica as predicated of many items differing in species in the ‘what sort of thing is it ?’ that can be found in Porphyry’s Isagoge. Two places of the reworking of Porphyry’s Isagoge (Madhal) at the beginning of Avicenna’s summa entitled Kitab al-Šifa’ will be mainly considered : the original account of differentia in chap. I, 13 ; and the rejection of Porphyry’s distinction between genus and differentia in chap. II, 1. By comparing these passages of Madhal to other sections of the Šifa’, it will be possible to reconstruct in a comprehensive way Avicenna’s refutation of the traditional account of differentia, in order to explain his preference for alternative definitions of this predicable in his other logical works. The exegetical problems posed by Porphyry’s definition of differentia, and the Greek and Arabic commentators’ ensuing discussions, provide the context in which Avicenna elaborates his refutation. Against this background, it will be argued that Avicenna is carrying out a refutation of the account of differentia first advanced by Porphyry and later adopted by the contemporary Peripatetic commentators active in Baghdad, in his effort to achieve a consistent interpretation of Aristotle’s claims on differentia.