 One important exception to this comes in the work of Samuel Alexander, who, in his 1920 work Space, Time and Deity develops a theory of categories in the realist spirit. Alexander defends a monist ontology in which he posits Space-Time as “the one monistic entity that encompasses every entity and every feature in reality” (Fisher 2015, 246). He sees the categories as grounded on the intrinsic nature of Space-Time, and posits as categorial features only those which are ‘pervasive’, that is, instantiated by every entity. The categories he identifies come in three ‘grades’ (or ranks of increasing complexity, in which the latter grades presuppose the former), giving us the following system: