In philosophy, ontology (from the Greek , genitive : of being (part. of : to be) and -λογία: science, study, theory) is the study of being or existence. This most fundamental branch of metaphysics seeks to describe or posit the basic categories and relationships of being or existence to define entities and types of entities within its framework. Ontology can be said to study conceptions of reality.
Some philosophers, notably of the Platonic school, contend that all nouns refer to entities. Other philosophers contend that some nouns do not name entities but provide a kind of shorthand way of referring to a collection (of either objects or events). In this latter view, mind, instead of referring to an entity, refers to a collection of mental events experienced by a person; society refers to a collection of persons with some shared characteristics, and geometry refers to a collection of a specific kind of intellectual activity. Any ontology must give an account of which words refer to entities, which do not, why, and what categories result. When one applies this process to nouns such as electrons, energy, contract, happiness, time, truth, causality, and God, ontology becomes fundamental to many branches of philosophy.
Ontologies :: Knowledge Representation

Buffalo Ontology Site - Information on ontology, on the history of ontology, and on contemporary ontology and its applications.
Descriptive and Formal Ontology - An extensive resource site on the development of ontology, especially in the twentieth century.
Meta Description: [ resource guide for contemporary ontology from a philosophical point of view: includes essays and bibliographies; provides a look at the theory's history and recent evolution ]
Metaphysics of Quality - Robert M. Pirsig deals with the fundamentals of existence and attempts to provide a more coherent system for understanding reality than current paradigms allow.
Meta Description: [ Robert M. Pirsig's MOQ deals with the fundamentals of existence and
provides a more coherent system for understanding reality than our
current paradigms allow ]
PhilSci Archive: Idealist Philosophy: What is Real ? - Paper by Axel Randrup defending idealism and arguing that materialism leads to logical contradictions, which closes by considering the implications of this for religion.
Qualia: The Knowledge Argument - Aims to establish that conscious experience involves non-physical properties. It is one of the most discussed arguments against physicalism; from the Stanford Encyclopedia by Martine Nida-Rümelin.
Stanford Encyclopedia of Philosophy: Abstract Objects - Survey of attempts to draw the distinction between concrete and abstract objects; by Gideon Rosen.
Stanford Encyclopedia of Philosophy: Events - Survey of philosophical views on the character and status of events; by Roberto Casati and Achille Varzi.
Stanford Encyclopedia of Philosophy: Existence - Survey article by Barry Miller.
Stanford Encyclopedia of Philosophy: Holes - An interesting case-study for ontologists and epistemologists.
Stanford Encyclopedia of Philosophy: Physicalism - Discussion of the thesis that everything is physical; by Daniel Stoljar.
Stanford Encyclopedia of Philosophy: The Identity of Indiscernibles - A principle of analytic ontology first explicitly formulated by Leibniz. It states that no two distinct substances exactly resemble each other.
Stanford Encyclopedia of Philosophy: Tropes - An article describing tropes; by John Bacon.
The Shift of Aspect in Language and Thought - Notes on the relational element in the assertion that things exist. Patterned on Wittgenstein's Philosophical Investigations.
What is Ontology? - Collected definitions, from leading philosophical reference works and from philosophers from Wolff to Husserl.
Meta Description: [ definitions of ontology by leading philosophers from christian wolff to edmund husserl ]
Why Does Anything Exist? - Sequence of short essays on this topic.
Meta Description: [ is Reality completely exhausted by some sort of natural ]
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