Philosophical Roots

The term ontology was taken from philosophy. According to Webster's Dictionary an ontology is

a branch of metaphysics relating to the nature and relations of being
a particular theory about the nature of being or the kinds of existence

Ontology (the "science of being") is a word, like metaphysics, that is used in many different senses. It is sometimes considered to be identical to metaphysics, but we prefer to use it in a more specific sense, as that part of metaphysics that specifies the most fundamental categories of existence, the elementary substances or structures out of which the world is made. Ontology will thus analyze the most general and abstract concepts or distinctions that underlay every more specific description of any phenomenon in the world, e.g. time, space, matter, process, cause and effect, system.

Recently, the term of "ontology" has been up taken by researchers in Artificial Intelligence, who use it to designate the building blocks out of which models of the world are made. An agent (e.g., an autonomous robot) using a particular model will only be able to perceive that part of the world that his ontology is able to represent. In this sense, only the things in his ontology can exist for that agent. In that way, an ontology becomes the basic level of a knowledge representation scheme. An example is set of link types for a semantic network representation which is based on a set of "ontological" distinctions: changing-invariant, and general-specific.

Ontology is originally a branch of philosophy that deals with the nature and the organization of reality. It tries to answer questions like "what is existence", "what properties can explain the existence" etc. Aristotle defined ontology as the science of being as such. Unlike the special sciences, each of which investigates a class of beings and their determinations, ontology regards "all the species qua being and the attributes that belong to it qua being". In this sense the philosophical ontology tries to answer the question "what is the being?" or, in a meaningful reformulation "what are the features common to all beings?".

This is what can be called "general ontology" - in contrast with various special ontologies for a particular domain. Such an ontology is not concerned with the existence of certain objects, but rather in the rigorous description of their forms of being, i.e., their structural features. In practice, ontology can be intended as the theory of the distinctions, which can be applied independently of the state of the world. In particular, we are interested in distinctions:

among the entities of the world (physical objects, events, regions...)
among the meta-level categories used to model the world (concept, property, quality, state, role, part...)

The research of ontology in this philosophical sense is relevant for knowledge based systems. Ontology forms a base for knowledge used in such systems.