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Ontology is the philosophical study of being. As one of the most fundamental concepts, being encompasses all of reality and every entity within it. To articulate the basic structure of being, ontology examines what all entities have in common and how they are divided into fundamental classes, known as categories. An influential distinction is between particular and universal entities. Particulars are unique, non-repeatable entities, like the person Socrates. Universals are general, repeatable entities, like the color green. Another contrast is between concrete objects existing in space and time, like a tree, and abstract objects existing outside space and time, like the number 7. Systems of categories aim to provide a comprehensive inventory of reality, employing categories such as substance, property, relation, state of affairs, and event.

Ontologists disagree about which entities exist on the most basic level. Platonic realism asserts that universals have objective existence. Conceptualism says that universals only exist in the mind while nominalism denies their existence. There are similar disputes about mathematical objects, unobservable objects assumed by scientific theories, and moral facts. Materialism says that, fundamentally, there is only matter while dualism asserts that mind and matter are independent principles. According to some ontologists, there are no objective answers to ontological questions but only perspectives shaped by different linguistic practices.

Ontology uses diverse methods of inquiry. They include the analysis of concepts and experience, the use of intuitions and thought experiments, and the integration of findings from natural science. Applied ontology employs ontological theories and principles to study entities belonging to a specific area. It is of particular relevance to information and computer science, which develop conceptual frameworks of limited domains. These frameworks are used to store information in a structured way, such as a college database tracking academic activities. Ontology is closely related to metaphysics and relevant to the fields of logic, theology, and anthropology.

The origins of ontology lie in the ancient period with speculations about the nature of being and the source of the universe, including ancient Indian, Chinese, and Greek philosophy. In the modern period, philosophers conceived ontology as a distinct academic discipline and coined its name.