 Categories deal with what is universal and necessary (AIntro12). Categories articulate in particular the Sosein (“being-such-and-such-a-way”) of entities; they specify configurations, structures and contents, not forms of existence (AIntro7). Categories specify the fundamental determinations of being, and are “principles” of being. As fundamental determinations of being, categories form the interior of entities, so to speak. In this sense, categories are immanent to the world, and do not form a second world (A16b). The categorial interior of entities can be viewed in terms of a core-periphery organization: the most fundamental categories structure the innermost core of entities, while other categories, such as scientific ones, add progressively more superficial layers. Ontological categories are the most fundamental, and form the network of internal, dynamic determinants and dependencies which articulate the furniture and structure of the world. Categories do not form a homogeneous continuum, but appear to be organized in groups (AIntro15). Some categories belong to all the spheres of being (ideal and real), some to the entire real world, others to a specific stratum of reality. The first group of categories, “modal categories,” is analyzed in detail in Possibility and Actuality; the second group, “fundamental categories,” is covered in The Structure of the Real World, while the third group, “special categories,” is explored in the Philosophy of Nature and other works. Fundamental categories include (1) categories organized in pairs of opposites, such as principle-concretum, element-complex, and form-matter; (2) strata categories, such as those that distinguish inanimate, living, psychological, and spiritual beings; and (3) the categories of intercategorial connections, or the group of categorial laws, such as the laws of coherence, stratification, and dependence among categories.