Handbook of the History of Logic: - Fordham University Faculty

300 Henrik Lagerlund
about differentia in the Categories. In Part II, Chapter 9, he, however, notes the
following:
We must not let anyone confound us by saying that rationality and
differentiae in general are present in a subject just as accidents are
present in a subject, their subjects being the things whose differentiae
they are; the analogy would be between the presence of rationality in
man and the presence of whiteness in body. For rationality is present
in a subject, that is, man, as part of it, while the relation of whiteness
to body is different. For this reason our description of accidents as
what are predicated in a subject should not be understood as meaning
that they are in a subject as a part of it, but rather that the subject
can exist without the accident. 49
Here he specifically takes up problem of whether the differentia is a substance or
a quality and he answers clearly that it is not a quality. Differentiae are not in
a subject, that is, the species, since they are part of the subject. A quality like
whiteness is in a subject and it is also something that the subject can lose without
ceasing to exist, but a differentia which is an essential part of it as a substance, it
cannot exist without.
The substantial form as the differentia is constitutive of the species and the
essence of it; something it naturally cannot exist without. One could then say
that the substantial form on Averroes view is a complement of the species as an
essential part of it. The distinction between divisive and constitutive differentiae
is going to be very important to understand Averroes’ view. The substantial form
is the thing in human beings that makes it onto what it is and to what the concept
‘differentia’ refers to in the definition of the species human being. In this respect it
is in the classification what divides the genus animal and also what constitute the
species human being. All other constitutive differentiae are going to be derivative
upon it as essential characteristics or complements of human beings. The view
expressed here can be found in Robert Kilwardby’s commentaries on the Isagoge
and the Categories (see below). It can also be read into Aquinas’ De ente et
essentia.
Averroes’ treatment of modal syllogistics is very interesting and strengthens his
general metaphysical interpretation of Aristotle’s logic. It seemed also to have
had some influence on the thirteenth century. 50 In his minor and very compact
Quaesita of the Prior Analytics, Averroes focuses his discussion mainly on the
modal syllogistic. In its interpretation of the modal syllogistic, this minor work
differs from the middle commentary, the Expositio. While the Expositio gives a
straight-forward temporal interpretation of the modalities, the Quaesita presents
a somewhat different view, one which is more ontological, and based on the nature
of the terms involved in the different syllogisms.
49Averroes, Middle commentary on Aristotle’s Categories, II, 9, 41-2.
50See Lagerlund [2000, Chapter 2], where the relation between Kilwardby and Averroes is
explained.