Ontology (from the Greek ὄν, genitive ὄντος: of being <neuter participle of εἶναι: to be> and -λογία: science, study, theory) is the philosophical study of the nature of being, existence or reality in general, as well as of the basic categories of being and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics, ontology deals with questions concerning what entities exist or can be said to exist, and how such entities can be grouped, related within a hierarchy, and subdivided according to similarities and differences.
Students of Aristotle first used the word 'metaphysica' (literally "after the physical") to refer to what their teacher described as "the science of being qua being" - later known as ontology. 'Qua' means 'in the capacity of': hence, ontology is inquiry into being in so much as it is being, or into being in general, beyond any particular thing which is or exists; and the study of beings insofar as they exist, and not insofar as, for instance, particular facts obtained about them or particular properties relating to them. Take anything you can find in the world, and look at it, not as a puppy or a slice of pizza or a folding chair or a president, but just as something that is. More specifically, ontology concerns determining what categories of being are fundamental and asks whether, and in what sense, the items in those categories can be said to "be".
Some philosophers, notably of the Platonic school, contend that all nouns (including abstract nouns) refer to existent entities. Other philosophers contend that nouns do not always name entities, but that some provide a kind of shorthand for reference 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. Between these poles of realism and nominalism, there are also a variety of other positions; but 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, space, time, truth, causality, and god, ontology becomes fundamental to many branches of philosophy.
While the etymology is Greek, the oldest extant record of the word itself is the Latin form ontologia, which appeared in 1606, in the work 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 OED (Oxford English Dictionary, second edition, 1989) appears in Bailey’s dictionary of 1721, which defines ontology as ‘an Account of being in the Abstract’ - though, of course, such an entry indicates the term was already in use at the time. It is likely the word was first used in its Latin form by philosophers based on the Latin roots, which themselves are based on the Greek. The current on-line edition of the OED (Draft Revision September 2008) gives as first occurrence in English a work by Gideon Harvey (1636/7-1702): Archelogia philosophica nova; or, New principles of Philosophy. Containing Philosophy in general, Metaphysicks or Ontology, Dynamilogy or a Discourse of Power, Religio Philosophi or Natural Theology, Physicks or Natural philosophy - London, Thomson, 1663.
Parmenides was among the first to propose an ontological characterization of the fundamental nature of reality. In his prologue or proem he describes two views of reality; initially that change is impossible, and therefore existence is eternal. Consequently our opinions about reality must often be false and deceitful. Most of western philosophy, and science - including the fundamental concepts of falsifiability and the conservation of energy - have emerged from this view. This posits that existence is what exists, and that there is nothing that does not exist. Hence, there can be neither void nor vacuum; and true reality can neither come into being nor vanish from existence. Rather, the entirety of creation is limitless, eternal, uniform, and immutable. Parmenides thus posits that change, as perceived in everyday experience, is illusory. Everything that we can apprehend is but one part of a single entity. This idea somewhat anticipates the modern concept of an ultimate grand unification theory that finally explains all of reality in terms of one inter-related sub-atomic reality which applies to everything.
The opposite of eleatic monism is the pluralistic conception of Being. In the 5th century BC, Anaxagoras & Leucippus replaced the reality of Being (unique and unchanging) with the Becoming and therefore by a more fundamental and elementary ontic plurality. This thesis originated in the Greek-ion world, stated in two different ways by Anaxagoras and by Leucippus. The first theory dealt with "seeds" (which Aristotle referred to as "homeomeries") of the various substances. The second was the atomistic theory, which dealt with reality as based on the vacuum, the atoms and their intrinsic movement in it.
The materialist Atomism proposed by Leucippus was indeterminist, but then developed by Democritus in determistic sense. It was later (IV century b.c) that the originary atomism was taken again as indeterministic by Epicurus. He confirmed the reality as composed of an infinity of indivisible, inchangeable corpuscles or atoms (atomon, lit. ‘uncuttable’), but he gives weight to characterize atoms while for Leucippus they are characterized by a "figure", an "order" and a "position" in the cosmos (Aristotle, Metaphysics, I , 4, 985). They are, besides, creating the whole with the intrinsic movement in the vacuum, producing the diverse flux of being. Their movement is influenced by the Parenklisis (Lucretius names it Clinamen) and that is determinated by the chance. These ideas foreshadowed our understanding of traditional physics until the nature of atoms was discovered in the 20th century.
Plato developed this distinction between true reality and illusion, in arguing that what is real are eternal and unchanging Forms or Ideas (a precursor to universals), of which things experienced in sensation are at best merely copies, and real only in so far as they copy (‘participate in’) such Forms. In general, Plato presumes that all nouns (e.g., ‘Beauty’) refer to real entities, whether sensible bodies or insensible Forms. Hence, in The Sophist Plato argues that Being is a Form in which all existent things participate and which they have in common (though it is unclear whether ‘Being’ is intended in the sense of existence, copula, or identity); and argues, against Parmenides, that Forms must exist not only of Being, but also of Negation and of non-Being (or Difference)..
Ontology as an explicit discipline was inaugurated by Aristotle, in his Metaphysics, as the study of that which is common to all things which exist, and of the categorisation of the diverse senses in which things can and do exist. What exists, in so far as Aristotle concludes, are a plurality of independently existing substances – roughly, physical objects – on which the existence of other things, such as qualities or relations, may depend; and of which substances consist both of a form (e.g. a shape, pattern, or organisation), and of a matter formed (Hylomorphism). Against Plato, who taught frameworks or the theory of forms, Aristotle holds that universals exist, these do not have an existence over and above the particular things which instantiate them.
A similar ontology was that proposed by the Stoics, where the being is necessitated by the Logos as Providence of a world totality known as God or Nature. The Stoics' philosophy may well be the first expression of Pantheism .
The principal questions of ontology are "What can be said to exist?" and "Into what categories, if any, can we sort existing things?" Various philosophers have provided different answers to this question.
One common approach is to divide the extant entities into groups called categories. Of course, such lists of categories differ widely from one another, and it is through the co-ordination of different categorial schemes that ontology relates to such fields as theology, library science and artificial intelligence.
Further examples of ontological questions include:
Quintessential ontological concepts include:
"What exists", "What is", "What am I", "What is describing this to me", all exemplify questions about being, and highlight the most basic problems in ontology: finding a subject, a relationship, and an object to talk about. During the Enlightenment the view of René Descartes that "I think therefore I am" generally prevailed. Descartes argued that "cogito ergo sum" could lead to a proof of the existence of God, using the ontological argument that had been formulated first by Anselm of Canterbury.
This answer, however, became increasingly unsatisfactory in the 20th century as the philosophy of mathematics and the philosophy of science and even particle physics explored some of the most fundamental barriers to knowledge about being. Sociological theorists, most notably George Herbert Mead and Erving Goffman, saw the Cartesian Other as a "Generalized Other," the imaginary audience that individuals use when thinking about the self. According to Mead, "we do not assume there is a self to begin with. Self is not presupposed as a stuff out of which the world arises. Rather the self arises in the world"  The Cartesian Other was also used by Sigmund Freud, who saw the superego as an abstract regulatory force, and Emile Durkheim who viewed this as a psychologically manifested entity which represented God in society at large.
Modern ontology includes both theories of levels and strata which are essential from a systemic as well as a philosophical viewpoint. Most prominent are the theories of Husserl and Poli that proposed respectively four and three main ontology levels; the latter theory considers three major levels of reality: material-energetic, biological and the mind (or psychological) level. These three levels of reality correspond to: (1) simple or complex, (2) super-complex and (3) ultra-complex systems; in terms of the corresponding levels of processes, super-complexity involves processes of component processes, while ultra-complexity involves meta-processes of super-complex process of further component processes.
Schools of subjectivism, objectivism and relativism existed at various times in the 20th century, and the postmodernists and body philosophers tried to reframe all these questions in terms of bodies taking some specific action in an environment. This relied to a great degree on insights derived from scientific research into animals taking instinctive action in natural and artificial settings — as studied by biology, ecology, and cognitive science.
The processes by which bodies related to environments became of great concern, and the idea of being itself became difficult to really define. What did people mean when they said "A is B", "A must be B", "A was B"...? Some linguists advocated dropping the verb "to be" from the English language, leaving "E Prime", supposedly less prone to bad abstractions. Others, mostly philosophers, tried to dig into the word and its usage. Heidegger attempted to distinguish being and existence. Heidegger suggests that our way of being human and the way the world is for us are given by the ontological assumptions that come along with our language. These assumptions provide the context for communication: a horizon of unspoken background meanings. Because these assumptions both generate and are regenerated in our everyday interactions, the locus of our way of being is the communicative event of language in use.
Social scientists adopt one of four main ontological approaches: realism (the idea that facts are out there just waiting to be discovered), empiricism (the idea that we can observe the world and evaluate those observations in relation to facts), positivism (which focuses on the observations themselves, attentive more to claims about facts than to facts themselves), and post-modernism (which holds that facts are fluid and elusive, so we should focus only on our observational claims).