The predicate must either be Genus, or Proprium, or Accident, of its subject.
Thus, if it be proprium of man to be a walking-biped, it must also be proprium of bird to be a flying-biped.
Thus, he may predicate biped as a proprium always belonging to man.
It is a proprium per se of man to be an animal by nature tractable.
Pulchr is not proprium of just; therefore, pulchrum ...