When one thing is predicated of another, all that which is predicable
of the predicate will be predicable also of the subject. Thus, ‘man’ is
predicated of the individual man; but ‘animal’ is predicated of ‘man’;
it will, therefore, be predicable of the individual man also: for the
individual man is both ‘man’ and ‘animal’.
If genera are different and co-ordinate, their differentiae are
themselves different in kind. Take as an instance the genus ‘animal’
and the genus ‘knowledge’. ‘With feet’, ‘two-footed’, ‘winged’,
‘aquatic’, are differentiae of ‘animal’; the species of knowledge are
not distinguished by the same differentiae. One species of knowledge
does not differ from another in being ‘two-footed’.
But where one genus is subordinate to another, there is nothing to
prevent their having the same differentiae: for the greater class is
predicated of the lesser, so that all the differentiae of the predicate
will be differentiae also of the subject.