Thus, B is the cause of A's inherence in the species of D:
consequently A must be of wider extent than B; otherwise why should
B be the cause of A's inherence in D any more than A the cause of
B's inherence in D? Now if A is an attribute of all the species of
E, all the species of E will be united by possessing some common cause
other than B: otherwise how shall we be able to say that A is
predicable of all of which E is predicable, while E is not
predicable of all of which A can be predicated? I mean how can there
fail to be some special cause of A's inherence in E, as there was of
A's inherence in all the species of D? Then are the species of E, too,
united by possessing some common cause? This cause we must look for.
Let us call it C.
We conclude, then, that the same effect may have more than one
cause, but not in subjects specifically identical. For instance, the
cause of longevity in quadrupeds is lack of bile, in birds a dry
constitution-or certainly something different.
18
If immediate premisses are not reached at once, and there is not
merely one middle but several middles, i.e. several causes; is the
cause of the property's inherence in the several species the middle
which is proximate to the primary universal, or the middle which is
proximate to the species? Clearly the cause is that nearest to each
species severally in which it is manifested, for that is the cause
of the subject's falling under the universal.
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