 Kilwardby thinks that sentences per se should be understood following An. Post. I.4–6, where Aristotle discusses four different notions of per se (kath’ hauto) predication, though Kilwardby seems only to have the first two in mind when discussing per se necessity. Aristotle says that the first type of per se predication (per se primo modo) occurs when the definition of the subject includes the predicate. The second type of per se predication (per se secundo modo) occurs when the definition of the predicate includes the subject. The best characterization of the first type is the genus/species relation, where the definition of a species includes its genus. The second type is often characterized by a proprium (property), since a proprium is included in the definition of a subject, as in ‘a human being is able to laugh’, where the term ‘human being’ is included in the definition of the predicate ‘able to laugh’. A sentence is per se necessary if it involves either of these two predications, according to Kilwardby. Necessity per accidens belongs to all other necessity sentences, which lack this intrinsic relation between subject and predicate.