Parmenides was among the first to propose an ontological characterization of the fundamental nature of reality. Ontology is the philosophical study of the nature of

• being
• becoming,
• existence
• reality

• what entities exist or can be said to exist
• how such entities can be grouped, related within a hierarchy,
•  subdivided according to similarities and differences

Some philosophers 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.

1. mind, instead of referring to an entity, refers to a collection of mental events experienced by a person;
2. society refers to a collection of persons with some shared characteristics, and
3. geometry refers to a collection of a specific kind of intellectual activity.^[1]

Some fundamental questions

Principal questions of ontology include:

• “What can be said to exist?”
• “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?”

One common approach is to divide the extant subjects and predicates into groups called categories. The categories are, properly speaking,^[2] the ways in which a being can be addressed simply as a being, such as

• what it is (its ‘whatness’, quidditas or essence)
• how it is (its ‘howness’ or qualitativeness), how much it is (quantitativeness)
• where it is
• its relatedness to other beings, etc

Further examples of ontological questions include:^{[citation needed]}

• 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?
• 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?

Concepts

Essential ontological dichotomies include:

• universals and particulars
• substance and accident
• abstract and concrete objects
• essence and existence
• determinism and indeterminism

Types

Philosophers can classify ontologies in various ways using criteria such as the degree of abstraction and field of application:^[3]

1. Upper ontology: concepts supporting development of an ontology, meta-ontology
2. Domain ontology: concepts relevant to a particular topic or area of interest, for example, information technology or computer languages, or particular branches of science
3. Interface ontology: concepts relevant to the juncture of two disciplines
4. Process ontology: inputs, outputs, constraints, sequencing information, involved in business or engineering processes

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.^[11] 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.^[14]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 ‘wholeness’ of human being itself.^[15]How to determine the ‘fitness’ of a ‘language’ to the world then becomes a subject for investigation.

Reality and actuality

According to A.N. Whitehead, for ontology, it is useful to distinguish the terms ‘reality’ and ‘actuality’.

• 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 can 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 of course 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.^[22

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 L Leibniz’ monads 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 are therefore have something behind them.^[23]
• 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. 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’.

Ontology (information science)