WHETHER THE SUBSTANTIAL FORM IS THE PRINCIPLE OF INDIVIDUATION OF MATERIAL SUBSTANCES 1
1. There is another important view on this matter: The internal principle of individuation is the substantial form. This is usually attributed to Durandus, On II [of the Sentences], dist. 3, q.2,2 although he does not exactly affirm it, as I shall say later. Averroes, however, seems to have taught it in [his commentary to Aristotle's] On the Soul I, Ch. 7, and Bk. II, at the beginning, and in comments 7, 8 and 9, and 60,3 and in [the commentary to] Physics III, comment 60, and Bk. IV, comment 38;4 Avicenna is also cited, Book of Nature VI, Part I, as saying that form "gives numerical unity to the subject."5 Zimara holds it, Theorem 97,6 and [so does] Sebastian, Bishop of Osma, On the Soul II, Ch. 1.7 Moreover, Aristotle favors it in the same place, when he says that "form is what constitutes 'this something.'"8 And the primary basis of this view is to be taken from this text of Aristotle, for the principle of individuation must be what  intrinsically constitutes 9 this substance and  is most proper to it. Therefore, by reason of the former property , it must be something substantial; for accidents, as it has been often said, do not constitute substance or this substance, for this substance, even [insofar] as it is "a this," is a being by itself and substantial. Moreover, by reason of the latter property , this principle cannot be matter, but form, because this matter is not most proper to this individual, since it could be under other forms. Therefore, form is the principle of individuation.