In metaphysics, ontology is the philosophical study of being, as well as related concepts such as existence, becoming, and reality. Ontology addresses questions like how entities are grouped into and which of these entities exist on the most fundamental level. Ontologists often try to determine what the categories or highest kinds are and how they form a system of categories that encompasses the classification of all entities. Commonly proposed categories include substances, properties, relations, states of affairs, and events. These categories are characterized by fundamental ontological concepts, including particularity and universality, abstractness and concreteness, or possibility and necessity. Of special interest is the concept of ontological dependence, which determines whether the entities of a category exist on the most fundamental level. Disagreements within ontology are often about whether entities belonging to a certain category exist and, if so, how they are related to other entities. When used as a countable noun, the words ontology and ontologies refer not to the science of being but to theories within the science of being. Ontological theories can be divided into various types according to their theoretical commitments. Monocategorical ontologies hold that there is only one basic category, but polycategorical ontologies rejected this view. Hierarchical ontologies assert that some entities exist on a more fundamental level and that other entities depend on them. Flat ontologies, on the other hand, deny such a privileged status to any entity. Classical compound The compound word ontology ('study of being') combines onto- (Greek: ὄν; () ὄντος) and logia (-λογία). While the etymology is Greek, the oldest extant records of the word itself is a Neo-Latin form ontologia, which appeared in 1606 in the Ogdoas Scholastica by Jacob Lorhard (Lorhardus), and in 1613 in the Lexicon philosophicum by Rudolf Göckel (Goclenius). The first occurrence in English of ontology, as recorded by the Oxford English Dictionary, came in 1664 through Archelogia philosophica nova.