Handbook of the History of Logic: - Fordham University Faculty

The Assimilation of Aristotelian and Arabic Logic up to the Later Thirteenth Century 297
close to the original text and only on occasion deviate in order to explain further
or to rule out misunderstandings. He also sometimes says that some argument or
view is wrong, but he does not bother to explain why or give the counter argument.
As an interpreter one therefore must really drag his view out from the text since
the text itself gives us very little help.
Averroes’ commentary on the part of Isagoge contains an interesting discussion
of differentiae. It starts by dividing up differentia into different senses. He begins
by saying that there are three senses of differentiae, namely a general, a proper
and a most proper. A general differentia is when one thing differs from another
as Socrates is another substance than Plato and when the same thing differs from
itself by a state or a disposition as, for example, the young Socrates differ from the
old. A proper differentia is when one thing differs from another by an inseparable
accident. The most proper differentia is a differentia that produces a difference
in species. Averroes calls the latter a species-producing differentia and the two
former differentiae without any qualification. According to standard terminology
what Averroes’s calls species-producing differentia is specific differentia.
As he explains, species-producing differentiae produce another being while the
other only produce diversity and ‘it is through those that produce another being
that genera are divided into species and that definitions are completed. The other
kind of differentiae produces diversity only in respect to the states of things, not
in respect to their essence.’
There is also another division among differentiae, namely they are either separable
or inseparable and the inseparable are divided into substantial (per se)and
accidental (per accidens). This division is supposed to map onto the division above
so that we get:
General differentia = Separable differentia.
Proper differentia = Inseparable accidental differentia.
Most proper differentia = Inseparable substantial differentia.
This means of course that the inseparable but accidental differentiae, like snubness
of nose, are not species-producing (specific) differentiae and hence not employed
in the definition of the essence of a thing. He explains:
The inseparable, in turn, are of two sorts, substantial (per se) and accidental
(per accidens). An example of the accidental sort is snubness of
nose, and examples of the substantial sort are rationality and capacity
for science in man. It is the substantial differentiae that are employed
in the definition of the essence of the thing to which they belong, and,
as we have said, it is they that produce another being by the side of the
genus to which they are joined. Differentiae that are accidental are not
employed in the definition of the essence of the thing of which they are
predicated, and do not produce another species, but only diversity. 45
45 Averroes, Middle Commentary on Porphyry’s Isagoge, 14.