 As in Lowe’s ontology, natural kinds play a fundamental role in the ontology of Brian Ellis (2001, 2002). For Ellis the term ‘natural kind’ covers three hierarchical categories: natural kinds of objects (substantive kinds), natural kinds of processes and events (dynamic kinds), and natural kinds of properties and relations (property kinds). The chemical elements and compounds are substantive kinds; the different chemical reactions are dynamic kinds; a tetrahedral molecular structure is a (structural) property kind (whose species include the particular tetrahedral structures of methane, carbon tetrachloride, silicon tetrafluoride and so forth). Ellis is uncommitted on whether the dynamic kinds are independent of the other two types of kinds; but these are independent of one another. Each kind is a universal, and the three categories of kind therefore form three categories of universal. Thus the substantive kinds—the familiar kinds of thing and stuff—are universals, but these are not reducible to property universals. Ellis notes that laws of nature have a hierarchical structure: Kepler’s laws of planetary motion are an application of Newton’s laws. He argues (2001, 90) that the only satisfactory account of this is fact that natural kinds also exist in hierarchies (see Criterion 5).