亞里斯多德根據是否表現本質（to ti esti）和能否跟主詞換位這兩條原則把謂詞分為四類：偶性（accident）、種（genus）、特性（property）和定義（definition）。這種劃分預設了屬（species）為主詞。可實際上，屬也可作個別事物的謂詞，就此而言，四謂詞之一的種應改為屬種。從謂詞劃分所操作的詞語對象上看，偶性和屬種是非複合的詞語，而特性和定義是複合的詞語，後者可以由前者的複合來說明。因此，謂詞在最抽象的層次上不應該分為四類，而應該分為兩類，即屬種和偶性。特性和定義是亞里斯多德不自覺地引入了兩個輔助原則對兩類謂詞加以複合的結果：引入模態原則將偶性劃分為必然偶性和偶然偶性，特性和定義都是種和必然偶性的複合物；引入屬種統一性原則來區分特性和定義——凡是在逐層劃 分事物 的過程中能 保持屬種 統一性的 必然偶性 就是差（differentia），而差和種的複合物就是定義。可是，由於任何偶性都有自己的屬種統一性，因而亞里斯多德所說的差是無法從其諸原則中推得的。

Based the capacities to express the nature (to ti esti) and
change position with the subjects, Aristotle divides predicates into for categories: accident, genus, property and definition. In this division, species is assumed to be a subject. Yet, in fact, “species” can also be a predicate. In this regard, genus, one of the four predicates, should be changed to genus-species. Considering the division of predicates,accident and genus-species are non-compound phrases; property and definition are compound phrases, while the latter can be explained by the compoundness of the former. Therefore, on the most abstract level, predicates should be divided into two rather than four categories, and the two categories are genus-species and accident. Property and definition are the results of the fact that Aristotle unconsciously introduces two subsidiary principles to compound the two categories of predicates.He introduces the modal principle to divide accident into necessary accident and accidental accident, while both property and definition are
compounds of genus and necessary accident; the unitary principle of genus-species was introduces to separate property from definition—the necessary accident that could maintain the unitary principle of genus-species in the process of dividing things level by level is differentia, while the compound of differentia and genus is definition. But, since any accident has its own genus-species unity, Aristotle's idea of differentia can not be inferred from his principles.