Notes: Ontology

Douglas Adams, I find, has always has good advice…even when i feel like an infinitely morose robot… ;D

  1. possible
  2. conceptual
  3. intended
  • Is existence a property?
  • How do the properties of an object relate to the object itself?

Build on XML’s ability to define customized tagging schemes and RDF’s flexible approach to representing data.

The first level above RDF required for the Semantic Web is an ontology language what can formally describe the meaning of terminology used in Web documents; beyond the basic semantics of RDF Schema.

  • XML provides a surface syntax for structured documents, but imposes no semantic constraints on the meaning of these documents.
  • XML Schema is a language for restricting the structure of XML documents and also extends XML with datatypes.
  • RDF is a datamodel for objects (“resources”) and relations between them, provides a simple semantics for this datamodel, and these datamodels can be represented in an XML syntax.
  • RDF Schema is a vocabulary for describing properties and classes of RDF resources, with a semantics for generalization-hierarchies of such properties and classes.
  • OWL adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. “exactly one”), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.

http://www.w3.org/2003/08/owlfaq

Q. What does the acronym “OWL” stand for?

A. Actually, OWL is not a real acronym. The language started out as the “Web Ontology Language” but the Working Group disliked the acronym “WOL.” We decided to call it OWL. The Working Group became more comfortable with this decision when one of the members pointed out the following justification for this decision from the noted ontologist A.A. Milne who, in his influential book “Winnie the Pooh” stated of the wise character OWL:

“He could spell his own name WOL, and he could spell Tuesday so that you knew it wasn’t Wednesday…”

Ontologies are often equated with taxonomic hierarchies of classes, class definitions, and the subsumption relation, but ontologies need not be limited to these forms. Ontologies are also not limited to conservative definitions — that is, definitions in the traditional logic sense that only introduce terminology and do not add any knowledge about the world.[11] To specify a conceptualization, one needs to state axioms that do constrain the possible interpretations for the defined terms.[1]

http://www.ontotext.com/

ontology in a functional frame.

not gonna say i disagree.

Werner Ceusters has noted the confusion caused by the significant differences in the meaning of word ontology when used by philosophy compared with the use of the word ontology in computer science, and advocates for greater precision in use of the word ontology so that members of the various disciplines using various definitions of the word ontology can communicate. He writes ‘before one is able to answer the question ‘what is an ontology?’, one must provide first an answer to the question ‘what does the word ontology mean?’.[58]

Pizza has base?

  •  RDF/XML is the normative syntax and should be used to exchange information between systems.
  • Notice that ontology does not have to express all the possible constraints – the level of details in conceptualization depends on the requirements of the intended application and expressing conceptualization in ontology in addition depends on the used ontology language.
  • Every knowledge base, knowledge-based system or agent is committed to some conceptualization, explicitly or implicitly. For these systems, what “exists” is what can be represented.

http://www.obitko.com/tutorials/ontologies-semantic-web/owl-dl-semantics.html

Ontology

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

  • 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:

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)

  1. set of types
  2. set of properties
  3. set of relationship types

There is also generally an expectation that the features of the model in an ontology should closely resemble the real world (related to the object).[3] What many ontologies have in common in both computer science and in philosophy is the representation of entities, ideas, and events, along with their properties and relations, according to a system of categories. According to Gruber (1993):

Ontologies are often equated with taxonomic hierarchies of classes, class definitions, and the subsumption relation, but ontologies need not be limited to these forms. Ontologies are also not limited to conservative definitions — that is, definitions in the traditional logic sense that only introduce terminology and do not add any knowledge about the world.[11] To specify a conceptualization, one needs to state axioms that do constrain the possible interpretations for the defined terms.[1]

Components

Contemporary ontologies share many structural similarities, regardless of the language in which they are expressed. As mentioned above, most ontologies describe individuals (instances), classes (concepts), attributes, and relations. In this section each of these components is discussed in turn. Common components of ontologies include:

  • Individuals: instances or objects (the basic or “ground level” objects)
  • Classes: sets, collections, concepts, classes in programmingtypes of objects, or kinds of things
  • Attributes: aspects, properties, features, characteristics, or parameters that objects (and classes) can have
  • Relations: ways in which classes and individuals can be related to one another
  • Function terms: complex structures formed from certain relations that can be used in place of an individual term in a statement
  • Restrictions: formally stated descriptions of what must be true in order for some assertion to be accepted as input
  • Rules: statements in the form of an if-then (antecedent-consequent) sentence that describe the logical inferences that can be drawn from an assertion in a particular form
  • Axioms: assertions (including rules) in a logical form that together comprise the overall theory that the ontology describes in its domain of application. This definition differs from that of “axioms” in generative grammar and formal logic. In those disciplines, axioms include only statements asserted as a priori knowledge. As used here, “axioms” also include the theory derived from axiomatic statements
  • Events: the changing of attributes or relations

Ontologies are commonly encoded using ontology languages.

OWL DL Semantics

Let us illustrate the use of OWL vocabulary on an example ontology (inspired by OWL Pizzas): “Pizza has PizzaBase as its base; Pizza is disjoint with PizzaBase; NonVegetarianPizza is exactly Pizza that is not VegetarianPizza; isIngredientOf is a transitive property; isIngredientOf is inverse of hasIngredient”. The example expressed in the description logic syntax follows: owl pizza The same example expressed using OWL Abstract Syntax formulates the same information using LISP-like notation, and in addition uses URI for identification of all classes and properties:

When embedding the example OWL ontology to RDF, every statement must be converted to triples – see the figure below. For example, the ∃R.C restriction is formed by anonymous resource of type owl:Restriction. This anonymous resource (blank node) is a subject for two properties owl:onProperty and owl:someValuesFrom that relate the restriction relation (property) and concept (class). The anonymous resource is then used to be related to the constrained class (by rdfs:subClassOf in our case). The example expressed in triples and serialized in N3 follows:

pizza owl rdf graph

Pizza OWL ontology expressed in RDF triples

The OWL DL uses all the SHOIN(D) features. The overview of the possible descriptions, data ranges, properties, individuals and data values is shown in the table in the previous pageThe DL description of the semantics was introduced in one of the previous sections. The domain of individuals in the model is ΔI, the domain of data values ΔID was added to specify semantics of data ranges. The ontology is formed by constraints on a model. The axioms that can be used to constrain a model are summarized in the table in the previous page. In addition to the standard description logic features there are so called annotation properties added. In addition to RDFS annotation properties (such as rdfs:comment andrdfs:label) there are properties that allow to state for example version information, state compatibility or incompatibility between ontologies. There is also a constructowl:imports