Can There Be Two Perfectly Identical Complexions? Peter of Abano and Jacopo of Forlì on Avicenna’s Interdict

Avicenna, in Book I of the Canon, within the context of his general doctrine of complexion, presents the eight modes of equality (modi aequalitatis) that concern specific or individual complexions. There he states quite clearly that each individual within the human species possesses a complexion that belongs to that individual exclusively, and with which it is impossible to associate another individual. In this contribution, after a brief survey of some features of the discussions around the individuality and unrepeatability of complexions in the physiognomic tradition (with particular reference to twins), I will consider two different takes on this Avicennian interdict: the first, which echoes the Persian philosopher and physician, is proposed by Peter of Abano in his Conciliator; the other is by Jacopo of Forlì, who, in his Questiones super primam et secundam fen primi Canonis Avicenne, seems to cautiously distance himself from Avicenna’s authority. The contrast between these two medieval responses allows us to grasp the importance of complexion as a theoretical tool for explaining differences between individuals of the same species – differences which are not merely random or accidental, nor strictly formal.