Part 3

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'.

When a term is applied to a subject, all terms that apply to that term also apply to the subject. For example, 'man' applies to an individual man. But 'animal' applies to 'man', so 'animal' also applies to the individual man. This is because 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'.

If genera are different and at the same level, their distinguishing features are different in kind. Take, for example, the genus 'animal' and the genus 'knowledge'. 'With feet', 'two-footed', 'winged', 'aquatic', are distinguishing features of 'animal'. The types of knowledge are not distinguished by these same features. One type of knowledge does not differ from another by 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.

But when one genus falls under another, nothing prevents both from having the same distinguishing features. This is because the broader class applies to the narrower class. So all the distinguishing features of the broader class will also be distinguishing features of the narrower class.