 The two main aspects of categories are their generality and their character of determination. This second feature is what makes them principles. Principles exhaust themselves in this determining role. They only exist for something else; they are something only with respect to the concretum that they determine and are in. Principles are nothing without their concretum, and the concretum cannot exist without its principles (A1a; 6b; 16b; and elsewhere). The ontological aspect of the categories consists in some kind of determination of their concreta. Categories as principles are independent from their concreta, but not from other categories (A11c). Principles imply one another, and all the categories characterizing a stratum of reality work together (A15c). Categories are far from being the only principles of entities. There are also highly particular principles structuring specific domains of being—such as natural laws or psychological laws—which are concreta with respect to general categories (A25f). There is a gradation of principles from the most general categories to specific real cases. Empirical laws are concreta with respect to general principles, and they are principles with respect to individual instances.