in the case of primary substances, e.g. curvature and the
essence of curvature if this is primary. (By a 'primary' substance I
mean one which does not imply the presence of something in something
else, i.e. in something that underlies it which acts as matter.) But
things which are of the nature of matter, or of wholes that include
matter, are not the same as their essences, nor are accidental unities
like that of 'Socrates' and 'musical'; for these are the same only
by accident.
12
Now let us treat first of definition, in so far as we have not
treated of it in the Analytics; for the problem stated in them is
useful for our inquiries concerning substance. I mean this
problem:-wherein can consist the unity of that, the formula of which
we call a definition, as for instance, in the case of man, 'two-footed
animal'; for let this be the formula of man. Why, then, is this one,
and not many, viz. 'animal' and 'two-footed'? For in the case of 'man'
and 'pale' there is a plurality when one term does not belong to the
other, but a unity when it does belong and the subject, man, has a
certain attribute; for then a unity is produced and we have 'the
pale man'. In the present case, on the other hand, one does not
share in the other; the genus is not thought to share in its
differentiae (for then the same thing would share in contraries; for
the differentiae by which the genus is divided are contrary). And even
if the genus does share in them, the same argument applies, since
the differentiae present in man are many, e.
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