  The third group is ontological; the issues here involve the place that categories occupy in the map of all things that exist or can exist. Ontology is the sub-discipline of philosophy that deals with being and what exists. The three most common questions raised in this context are: (1) Are categories extra-mental entities, such as properties, qualities, relations, structures, sets, classes, or forms? (2) Are categories intra-mental entities, such as phenomena, mental acts, mental contents, Gestalten, or mental structures? (3) Are categories linguistic entities, such as meanings, words, types, tokens, predicates, or syntactical places? Four views stand out. According to one, categories are linguistic entities–call them words–such as the words ‘quality’ and ‘relation,’ with which we speak about things. According to another, categories are mental acts–call them concepts–such as the concept of quality or the concept of relation, with which we think about things. Categories are also thought to be extra-mental features that things about which we think and speak have–call them properties–such as the properties of being a quality or of being a relation. Finally, there is an inclusive view that tries to integrate all of these three positions into one, arguing that categories are words, concepts, and properties but in different ways.  These four views are not the only choices available (see Gracia 1999), but they are the most popular views considered in the Middle Ages.