 The idea is here that since ‘human being’ is not predicated per se of its subject ‘literate being’, the sentence (5:1) is not a per se necessity sentence and therefore not convertible. (5:1) is a necessity sentence, though of the per accidens type, since it is necessarily true only in the sense that being human and being literate are not separable. Kilwardby implies that the relation between the subject and predicate terms must be of a special kind if a sentence is to be called necessary per se. In (5:1), ‘literate being’ and ‘human being’ do not have the close per se relation Kilwardby demands of a convertible sentence.