 Husserl’s material categories, by contrast, classify entities according to their nature or essence, with the highest material genera to be arrived at by a process of generalization to the most general kind of content involved, rather than by the formalization that involves an emptying of all content (1913 [1962], 65). The highest material categories are the three regions: nature (including physical objects and events), culture (including artifacts, social entities, and values), and consciousness (cf. Smith 2004). While formal and material category systems each form a hierarchy (1913 [1962], 64), considered jointly their categories are not mutually exclusive, since one and the same entity may be categorized either in terms of its material nature or its form. For further discussion of Husserl’s categories, see Smith (2007, 135–161).