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.
