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Ontology is the branch of philosophy that studies concepts such as existence, being, becoming, and reality. It includes the questions of how entities are grouped into basic categories and which of these entities exist on the most fundamental level. Ontology is traditionally listed as a part of the major branch of philosophy known as metaphysics.
The compound word ontology ('study of being') combines
- onto- (Greek: ὄν, on; [i] GEN. ὄντος, ontos, 'being' or 'that which is') and
- -logia (-λογία, 'logical discourse').
- 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... by Gideon Harvey The word was first used, in its Latin form, by philosophers, and based on the Latin roots (and in turn on the Greek ones).
Some philosophers, notably in the traditions 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 either of objects or of 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 specific kinds of intellectual activities.[need quotation to verify] Between these poles of realism and nominalism stand a variety of other positions.
Some fundamental questionsEdit
Principal questions of ontology include
- "What can be said to exist?"
- "What is a thing?"
- "Into what categories, if any, can we sort existing things?"
- "What are the meanings of being?"
- "What are the various modes of being of entities?"
Various philosophers have provided different answers to these questions. One common approach involves dividing the extant subjects and predicates into groups called categories. Such lists of categories differ widely from one another, and it is through the co-ordination of different categorical schemes that ontology relates to such fields as library science and artificial intelligence. Such an understanding of ontological categories, however, is merely taxonomic, classificatory. Aristotle's categories are the ways in which a being may be addressed simply as a being, such as:
- what it is (its 'whatness', quiddity, haecceity or essence);
- how it is (its 'howness' or qualitativeness);
- how much it is (quantitativeness);
- where it is (its relatedness to other beings).
Further examples of ontological questions include
- What is existence, i.e. what does it mean for a being to be?
- Is existence a property?
- Is existence a genus or general class that is simply divided up by specific differences?
- Which entities, if any, are fundamental?
- Are all entities objects?
- How do the properties of an object relate to the object itself?
- Do physical properties actually exist?
- What features are the essential, as opposed to merely accidental attributes of a given object?
- How many levels of existence or ontological levels are there? And what constitutes a "level"?
- What is a physical object?
- Can one give an account of what it means to say that a physical object exists?
- Can one give an account of what it means to say that a non-physical entity exists?
- What constitutes the identity of an object?
- When does an object go out of existence, as opposed to merely changing?
- Do beings exist other than in the modes of objectivity and subjectivity, i.e. is the subject/object split of modern philosophy inevitable?
Flat vs ordered vs sortedEdit
Jonathan Schaffer distinguishes between three types of ontologies: flat, sorted and ordered.
Flat ontologies are only interested in the difference between existence and non-existence. They are flat because each flat ontology can be represented by a simple set containing all the entities to which this ontology is committed. An influential exposition of this approach comes from Willard Van Orman Quine which is why it has been referred to as the Quinean approach to meta-ontology. This outlook doesn't deny that the existing entities can be further subdivided and may stand in various relations to each other. These issues are questions for the more specific sciences, but they don't belong to ontology in the Quinean sense.
Sorted ontologies are concerned with the categories of being. Each sorted ontology posits a number of categories. These categories are exclusive and exhaustive: every existing entity belongs to exactly one category. A recent example of a sorted ontology is E.J. Lowe's four-category-ontology. The four categories are object, kind, mode and attribute. The fourfold structure is based on two distinctions. The first distinction is between substantial entities (objects and kinds) and non-substantial entities (modes and attributes). The second distinction is between particular entities (objects and modes) and universal entities (kinds and attributes). Reality is built up through the interplay of entities belonging to different categories: particular entities instantiate universal entities, and non-substantial entities characterize substantial entities.
Ordered ontologies are interested in the degree of fundamentality of the entities they posit. Their main goal is to figure out which entities are fundamental and how the non-fundamental entities depend on them. The concept of fundamentality is usually defined in terms of metaphysical grounding. Fundamental entities are different from non-fundamental entities because they are not grounded in other entities. For example, it is sometimes held that elementary particles are more fundamental than the macroscopic objects (like chairs and tables) they compose. This is a claim about the grounding-relation between microscopic and macroscopic objects. Schaffer's priority monism is a recent form of an ordered ontology. He holds that on the most fundamental level there exists only one thing: the world as a whole. This thesis doesn't deny our common-sense intuition that the distinct objects we encounter in our everyday affairs like cars or other people exist. It only denies that these objects have the most fundamental form of existence. An example of an ordered ontology in continental philosophy comes from Nicolai Hartmann. He asserts that reality is made up of four levels: the inanimate, the biological, the psychological and the spiritual. These levels form a hierarchy in the sense that the higher levels depend on the lower levels while the lower levels are indifferent to the higher levels.
Thing ontologies vs fact ontologiesEdit
Thing ontologies and fact ontologies are one-category-ontologies: they both hold that all fundamental entities belong to the same category. They disagree on whether this category is the category of things or of facts. A slogan for fact ontologies comes from Ludwig Wittgenstein: "The world is the totality of facts, not of things".
One difficulty in characterizing this dispute is to elucidate what things and facts are, and how they differ from each other. Things are commonly contrasted with the properties and relations they instantiate. Facts, on the other hand, are often characterized as having these things and the properties/relations as their constituents. This is reflected in a rough linguistic characterization of this difference where the subjects and objects of an assertion refer to things while the assertion as a whole refers to a fact.
Reism in continental philosophy is one form of thing ontology. Franz Brentano developed a version of reism in his later philosophy. He held that only concrete particular things exist. Things can exist in two forms: either as spatio-temporal bodies or as temporal souls. Brentano was aware of the fact that many common-sense expressions seem to refer to entities that don't have a place in his ontology, like properties or intentional objects. This is why he developed a method to paraphrase these expressions in order to avoid these ontological commitments.
D. M. Armstrong is a well-known defender of fact ontology. He and his followers refer to facts as states of affairs. States of affairs are the basic building blocks of his ontology: they have particulars and universals as their constituents but they are primary in relation to particulars and universals. States of affairs have independent existence while "[u]npropertied particulars and uninstantiated universals are false abstractions".
Constituent ontologies vs blob theoriesEdit
Constituent ontologies and blob theories are concerned with the internal structure of objects. Constituent ontologies hold that objects have an internal structure made up of constituents. This is denied by blob theories: they contend that objects are structureless "blobs".
Bundle theories are examples of constituent ontologies. Bundle theorists assert that an object is nothing but the properties it "has". On this account, a regular apple could be characterized as a bundle of redness, roundness, sweetness, etc. Defenders of bundle theory disagree on the nature of the bundled properties. Some affirm that these properties are universals while others contend that they are particulars, so-called "tropes".
Class nominalism, on the other hand, is a form of blob theory. Class nominalists hold that properties are classes of things. To instantiate a property is merely to be a member of the corresponding class. So properties are not constituents of the objects that have them.
- Upper ontology: concepts supporting development of an ontology, meta-ontology.
- Domain ontology: concepts relevant to a particular topic, domain of discourse, or area of interest, for example, to information technology or to computer. languages, or to particular branches of science.
- Interface ontology: concepts relevant to the juncture of two disciplines.
- Process ontology: inputs, outputs, constraints, sequencing information, involved in business or engineering processes.
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Parmenides and monismEdit
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In the Greek philosophical tradition, Parmenides was among the first to propose an ontological characterization of the fundamental nature of existence. In the prologue (or proem) to On Nature, he describes two views of existence. Initially, nothing comes from nothing, thus existence is eternal. This posits that existence is what may be conceived of by thought, created, or possessed. Hence, there may be neither void nor vacuum; and true reality neither may come into being nor vanish from existence. Rather, the entirety of creation is eternal, uniform, and immutable, though not infinite (Parmenides characterized its shape as that of a perfect sphere). Parmenides thus posits that change, as perceived in everyday experience, is illusory. Everything that may be apprehended is but one part of a single entity. This idea somewhat anticipates the modern concept of an ultimate theory of everything that finally describes all of existence in terms of one inter-related sub-atomic reality which applies to everything. Most of western philosophy (especially the philosophy of Baruch Spinoza)—including the fundamental concepts of falsifiability—has emerged from this view.
Opposite to the Eleatic monism of Parmenides is the pluralistic conception of being. In the 5th century BC, Anaxagoras and Leucippus replaced the reality of being (unique and unchanging) with that of becoming, therefore by a more fundamental and elementary ontic plurality. This thesis originated in the Hellenic 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 Democritus (c. 460 – c. 370 BC) subsequently developed it in a deterministic way. Later (4th century BC), Epicurus took the original atomism again as indeterministic. He saw reality as composed of an infinity of indivisible, unchangeable corpuscles or atoms (from the Greek atomon, lit. 'uncuttable'), but he gives weight to characterize atoms whereas for Leucippus they are characterized by a "figure", an "order" and a "position" in the cosmos. Atoms 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 determined by chance. These ideas foreshadowed the understanding of traditional physics until the advent of 20th-century theories on the nature of atoms.[page needed]
Plato developed the 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 ("partake of") 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).
In his Categories, Aristotle (384–322 BCE) identifies ten possible kinds of things that may be the subject or the predicate of a proposition. For Aristotle there are four different ontological dimensions:
- according to the various categories or ways of addressing a being as such
- according to its truth or falsity (e.g. fake gold, counterfeit money)
- whether it exists in and of itself or simply 'comes along' by accident
- according to its potency, movement (energy) or finished presence (Metaphysics Book Theta).
According to Avicenna (c. 980 – 1037), and in an interpretation of Greek Aristotelian and Platonist ontological doctrines in medieval metaphysics, being is either necessary, contingent qua possible, or impossible. Necessary being is that which cannot but be, since its non-being would entail a contradiction. Contingent qua possible being is neither necessary nor impossible for it to be or not to be. It is ontologically neutral, and is brought from potential existing into actual existence by way of a cause that is external to its essence. Its being is borrowed – unlike the necessary existent, which is self-subsisting and impossible not to be. As for the impossible, it necessarily does not exist, and the affirmation of its being would involve a contradiction.
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Other ontological topicsEdit
The concept of ontological formations refers to formations of social relations understood as dominant ways of living. Temporal, spatial, corporeal, epistemological, and performative relations are taken to be central to understanding a dominant formation. That is, a particular ontological formation is based on how ontological categories of time, space, embodiment, knowing and performing are lived—objectively and subjectively. Different ontological formations include the customary (including the tribal), the traditional, the modern, and the postmodern. The concept was first introduced by Paul James in 2006, together with a series of writers including Damian Grenfell and Manfred Steger.
In the engaged theory approach, ontological formations are seen as layered and intersecting rather than singular formations. They are 'formations of being'. This approach avoids the usual problems of a Great Divide being posited between the modern and the pre-modern. From a philosophical distinction concerning different formations of being, the concept then provides a way of translating into practical understandings concerning how humans might design cities and communities that live creatively across different ontological formations, for example cities that are not completely dominated by modern valences of spatial configuration. Here the work of Tony Fry is important.
Ontology of fictional charactersEdit
- 'Nero worshipped (the god);'
- 'Mars, the god, does not exist;' and
- 'Eliza Doolittle, in George Bernard Shaw's Pygmalion, is a flower girl.'
According to Amie L. Thomasson, fictional discourse can be of four sorts:
- Uttered within works of fiction;
- Philosophical exercises such as 'Captain Marvel does not exist';
- Treating fictional characters as if they were 'real', such as 'Superman can leap tall buildings;' and
- Discourse about works of fiction, such as 'Professor Higgins was created by George Bernard Shaw'.
- the real: those that can be perceived, or can be inferred from perception.
- the fictitious: abstractions that referred to perceptible things.
- the fabulous: those that can be found only in the imagination, where the word 'exist' applies to such only in the sense that they do not really exist.
- the genuinely historical;
- the fictional;
- the real;
- the merely imagined;
- the existent; and
- the non-existent.
Alexius Meinong would put fictional entities into the category which he called subsistence. This category contains objects that neither exist spatially or non-spatially. However, they do have properties. The properties are given to these objects in the way they are said to be described. For example, we can talk about the tall unicorn even though the tall unicorn does not exist. We can say the unicorn is in fact tall because this follows from the properties in which the object is characterized.
Ontological and epistemological certaintyEdit
René Descartes, with cogito, ergo sum (je pense donc je suis, "I think, therefore I am"), argued that "the self" is something that we can know exists with epistemological certainty. Descartes argued further that this knowledge could lead to a proof of the certainty of the existence of God, using the ontological argument that had been formulated first by Anselm of Canterbury.
Certainty about the existence of "the self" and "the other", however, came under increasing criticism in the 20th century. 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 by Émile Durkheim, who viewed this as a psychologically manifested entity which represented God in society at large.
Body and environment, questioning the meaning of beingEdit
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. Martin Heidegger distinguished human being as existence from the being of things in the world. Heidegger proposes that our way of being human and the way the world is for us are cast historically through a fundamental ontological questioning. These fundamental ontological categories provide the basis for communication in an age: a horizon of unspoken and seemingly unquestionable background meanings, such as human beings understood unquestioningly as subjects and other entities understood unquestioningly as objects. Because these basic ontological meanings both generate and are regenerated in everyday interactions, the locus of our way of being in a historical epoch is the communicative event of language in use. For Heidegger, however, communication in the first place is not among human beings, but language itself shapes up in response to questioning (the inexhaustible meaning of) being. Even the focus of traditional ontology on the 'whatness' or quidditas of beings in their substantial, standing presence can be shifted to pose the question of the 'whoness' of human being itself.
Ontology and languageEdit
Some philosophers suggest that the question of "What is?" is (at least in part) an issue of usage rather than a question about facts. This perspective is conveyed by an analogy made by Donald Davidson: Suppose a person refers to a 'cup' as a 'chair' and makes some comments pertinent to a cup, but uses the word 'chair' consistently throughout instead of 'cup'. One might readily catch on that this person simply calls a 'cup' a 'chair' and the oddity is explained. Analogously, if we find people asserting 'there are' such-and-such, and we do not ourselves think that 'such-and-such' exist, we might conclude that these people are not nuts (Davidson calls this assumption 'charity'), they simply use 'there are' differently than we do. The question of What is? is at least partially a topic in the philosophy of language, and is not entirely about ontology itself. This viewpoint has been expressed by Eli Hirsch.
Hirsch interprets Hilary Putnam as asserting that different concepts of "the existence of something" can be correct. This position does not contradict the view that some things do exist, but points out that different 'languages' will have different rules about assigning this property. How to determine the 'fitness' of a 'language' to the world then becomes a subject for investigation.
Common to all Indo-European copula languages is the double use of the verb "to be" in both stating that entity X exists ("X is.") as well as stating that X has a property ("X is P"). It is sometimes argued that a third use is also distinct, stating that X is a member of a class ("X is a C"). In other language families these roles may have completely different verbs and are less likely to be confused with one another. For example they might say something like "the car has redness" rather than "the car is red." Hence any discussion of "being" in Indo-European language philosophy may need to make distinctions between these senses.
Ontology and human geographyEdit
In human geography there are two types of ontology: small "o" which accounts for the practical orientation, describing functions of being a part of the group, thought to oversimplify and ignore key activities. The other "o", or big "O", systematically, logically, and rationally describes the essential characteristics and universal traits. This concept relates closely to Plato's view that the human mind can only perceive a bigger world if they continue to live within the confines of their "caves". However, in spite of the differences, ontology relies on the symbolic agreements among members. That said, ontology is crucial for the axiomatic language frameworks.
Reality and actualityEdit
According to Alfred N. Whitehead, for ontology, it is useful to distinguish the terms 'reality' and 'actuality'. In this view, an 'actual entity' has a philosophical status of fundamental ontological priority, while a 'real entity' is one which may be actual, or may derive its reality from its logical relation to some actual entity or entities. For example, an occasion in the life of Socrates is an actual entity. But Socrates' being a man does not make 'man' an actual entity, because it refers indeterminately to many actual entities, such as several occasions in the life of Socrates, and also to several occasions in the lives of Alcibiades, and of others. But the notion of man is real; it derives its reality from its reference to those many actual occasions, each of which is an actual entity. An actual occasion is a concrete entity, while terms such as 'man' are abstractions from many concrete relevant entities.
According to Whitehead, an actual entity must earn its philosophical status of fundamental ontological priority by satisfying several philosophical criteria, as follows:
- There is no going behind an actual entity, to find something more fundamental in fact or in efficacy. This criterion is to be regarded as expressing an axiom, or postulated distinguished doctrine.
- An actual entity must be completely determinate in the sense that there may be no confusion about its identity that would allow it to be confounded with another actual entity. In this sense an actual entity is completely concrete, with no potential to be something other than itself. It is what it is. It is a source of potentiality for the creation of other actual entities, of which it may be said to be a part cause. Likewise it is the concretion or realization of potentialities of other actual entities which are its partial causes.
- Causation between actual entities is essential to their actuality. Consequently, for Whitehead, each actual entity has its distinct and definite extension in physical Minkowski space, and so is uniquely identifiable. A description in Minkowski space supports descriptions in time and space for particular observers.
- It is part of the aim of the philosophy of such an ontology as Whitehead's that the actual entities should be all alike, qua actual entities; they should all satisfy a single definite set of well stated ontological criteria of actuality.
Whitehead proposed that his notion of an occasion of experience satisfies the criteria for its status as the philosophically preferred definition of an actual entity. From a purely logical point of view, each occasion of experience has in full measure the characters of both objective and subjective reality. Subjectivity and objectivity refer to different aspects of an occasion of experience, and in no way do they exclude each other.
Examples of other philosophical proposals or candidates as actual entities, in this view, are Aristotle's 'substances', Leibniz' monads, and Descartes ′res verae' , and the more modern 'states of affairs'. Aristotle's substances, such as Socrates, have behind them as more fundamental the 'primary substances', and in this sense do not satisfy Whitehead's criteria. Whitehead is not happy with Leibniz' monads as actual entities because they are "windowless" and do not cause each other. 'States of affairs' are often not closely defined, often without specific mention of extension in physical Minkowski space; they are therefore not necessarily processes of becoming, but may be as their name suggests, simply static states in some sense. States of affairs are contingent on particulars, and therefore have something behind them. One summary of the Whiteheadian actual entity is that it is a process of becoming. Another summary, referring to its causal linkage to other actual entities, is that it is "all window", in contrast with Leibniz' windowless monads.
This view allows philosophical entities other than actual entities to really exist, but not as fundamentally and primarily factual or causally efficacious; they have existence as abstractions, with reality only derived from their reference to actual entities. A Whiteheadian actual entity has a unique and completely definite place and time. Whiteheadian abstractions are not so tightly defined in time and place, and in the extreme, some are timeless and placeless, or 'eternal' entities. All abstractions have logical or conceptual rather than efficacious existence; their lack of definite time does not make them unreal if they refer to actual entities. Whitehead calls this 'the ontological principle'.
There is an established and long philosophical history of the concept of atoms as microscopic physical objects. They are far too small to be visible to the naked eye. It was as recent as the nineteenth century that precise estimates of the sizes of putative physical atoms began to become plausible. Almost direct empirical observation of atomic effects was due to the theoretical investigation of Brownian motion by Albert Einstein in the very early twentieth century. But even then, the real existence of atoms was debated by some. Such debate might be labeled 'microcosmic ontology'. Here the word 'microcosm' is used to indicate a physical world of small entities, such as for example atoms.
Subatomic particles are usually considered to be much smaller than atoms. Their real or actual existence may be very difficult to demonstrate empirically. A distinction is sometimes drawn between actual and virtual subatomic particles. Reasonably, one may ask, in what sense, if any, do virtual particles exist as physical entities? For atomic and subatomic particles, difficult questions arise, such as do they possess a precise position, or a precise momentum? A question that continues to be controversial is 'to what kind of physical thing, if any, does the quantum mechanical wave function refer?'.
In the Western Christian tradition, in his 1078 work Proslogion, Anselm of Canterbury proposed what is known as 'the ontological argument' for the existence of God.[ii] Anselm defined God as "that than which nothing greater can be thought", and argued that this being must exist in the mind, even in the mind of the person who denies the existence of God. He suggested that, if the greatest possible being exists in the mind, it must also exist in reality. If it only exists in the mind, then an even greater being must be possible—one which exists both in the mind and in reality. Therefore, this greatest possible being must exist in reality. Seventeenth century French philosopher René Descartes deployed a similar argument. Descartes published several variations of his argument, each of which centred on the idea that God's existence is immediately inferable from a "clear and distinct" idea of a supremely perfect being. In the early eighteenth century, Gottfried Leibniz augmented Descartes' ideas in an attempt to prove that a "supremely perfect" being is a coherent concept. Norman Malcolm revived the ontological argument in 1960 when he located a second, stronger ontological argument in Anselm's work; Alvin Plantinga challenged this argument and proposed an alternative, based on modal logic. Attempts have also been made to validate Anselm's proof using an automated theorem prover.
More recently, Kurt Gödel proposed a formal argument for God's existence. Other arguments for God's existence have been advanced, including those made by Islamic philosophers Mulla Sadra and Allama Tabatabai.
Hintikka's locution for existenceEdit
- Anselm of Canterbury
- Thomas Aquinas
- David Malet Armstrong
- Alain Badiou
- Gustav Bergmann
- Roy Bhaskar
- Bernard Bolzano
- Franz Brentano
- Martin Buber
- Mario Bunge
- Rudolf Carnap
- Ernst Cassirer
- Gilles Deleuze
- Daniel Dennett
- Jacques Derrida
- René Descartes
- Fyodor Dostoevsky
- Maurizio Ferraris
- Michel Foucault
- Richard Foreman
- Hans-Georg Gadamer
- Étienne Gilson
- Nicolai Hartmann
- Georg Wilhelm Friedrich Hegel
- Martin Heidegger
- Heraclitus of Ephesus
- Jaakko Hintikka
- Edmund Husserl
- Roman Ingarden
- Immanuel Kant
- Leszek Kołakowski
- Julia Kristeva
- Susanne Langer
- Louis Lavelle
- Gottfried Leibniz
- Douglas Lenat
- Stanisław Leśniewski
- David Kellogg Lewis
- Emmanuel Levinas
- John Locke
- E.J. Lowe
- György Lukács
- Alexius Meinong
- Friedrich Nietzsche
- Keiji Nishitani
- Charles Sanders Peirce
- Karl Popper
- Proclus Lycaeus
- W.V.O. Quine
- Bertrand Russell
- Gilbert Ryle
- Mulla Sadra
- Jean-Paul Sartre
- Jonathan Schaffer
- Arthur Schopenhauer
- Duns Scotus
- John Searle
- Adi Shankaracharya
- Theodore Sider
- Peter Simons
- Barry Smith
- Baruch Spinoza
- Shahab al-Din Suhrawardi
- Peter van Inwagen
- Achille Varzi
- Gianni Vattimo
- Swami Vivekananda
- Alfred North Whitehead
- William of Ockham
- Ludwig Wittgenstein
- Edward N. Zalta
- Dean Zimmerman
- Slavoj Žižek
- Applied ontology
- Five Ws
- Foundation ontology
- Geopolitical ontology
- Guerrilla ontology
- Integrative level
- Living educational theory
- Modal logic
- Moral ontology
- Ontological paradox
- Philosophy of mathematics
- Philosophy of science
- Philosophy of space and time
- Physical ontology
- Porphyrian tree
- Quantum ontology
- Speculative realism
- Structure and agency
- Subject–object problem
- ὄν is the present-tense participle of the verb εἰμί (eimí, 'to be' or 'I am').
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