g. it is not all animal which is either
whole-winged or split-winged but all winged animal, for it is winged
animal to which this differentiation belongs. The primary
differentiation of animal is that within which all animal falls. The
like is true of every other genus, whether outside animal or a
subaltern genus of animal; e.g. the primary differentiation of bird is
that within which falls every bird, of fish that within which falls
every fish. So, if we proceed in this way, we can be sure that nothing
has been omitted: by any other method one is bound to omit something
without knowing it.
To define and divide one need not know the whole of existence. Yet
some hold it impossible to know the differentiae distinguishing each
thing from every single other thing without knowing every single other
thing; and one cannot, they say, know each thing without knowing its
differentiae, since everything is identical with that from which it
does not differ, and other than that from which it differs. Now
first of all this is a fallacy: not every differentia precludes
identity, since many differentiae inhere in things specifically
identical, though not in the substance of these nor essentially.
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