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reality: for Aristotle and many others, primarily objects and

processes. In the empirical 19th century, metaphysics was

perceived as experimentally unverifiable and, therefore, irre-

sponsible, and parts of it may have been so. There has been

no getting away, however, from the necessity of understand-

ing the basic categories underlying both human thinking and

the spectacular successes of empirical sciences. In the 20th

century, the concern about foundational issues of physicsand other hard sciences has become common and acute. The

term ontologywas used for a while as a politesubstitute for

metaphysics, but towards the end of the century, the later term

regained its respectability. Accordingly, ontology started

being used more correctly as the actual result of metaphysical

research, a specific system of basic concepts.

The major issue of metaphysics is the status of prop-

erties. The realists recognize the existence of both indi-

viduals, such as houses or cars, which are spatio-temporal

entities, and of their shared properties, such as costliness

or redness, which are not spatio-temporal but rather

abstract. Their opponents, the naturalists, recognize onlyindividuals. Both camps, existing in a large number of

variations, have a difficult philosophical issue to deal

with: for the realists, the different status of properties; for

the nominalists, the existence of those properties. For a

specific ontology, the matter is only important in one

sense, namely, whether the ontology is to contain proper-

ties or not, and the nature of any reasonable application

usually imposes the realist solution.

2.2.2 Formal ontologyFormal ontology (see Guarino 1995; 1998) was developed

late in the 20th century as a cross of mathematical disci-

plines of mereology, which studies the relations betweenparts and wholes, theory of dependence, and topology. But

it was originally Edmund Husserl, earlier in the century,

who saw a need to establish formal ontology as the theory

of things parallel to logic, the theory of truths.

The agenda of formal ontology typically includes

explorations into notions of time and space, modality,

especially the deontic (imperative, must-do) phenomena,

taxonomy of artifacts, and types of inheritance. A specific

ontology, in its implementation, has to take a position on

each of those and other issues of formal ontology, so the

awareness of these issues is essential for the developers.

2.2.3 Ontologies as engineering artifactsThe practical use of an ontology involves constructed reality

(see Gruber 1995). The MikroKosmos ontology, containing

over 7,000 actual and many more virtual concepts (i.e.

concepts derivable from the actual concepts with the help of

well-defined rules), was created by a team of computer

scientists and linguists over almost a decade at considerable

cost (see Nirenburg and Raskin 2004). After the original

time- and effort-consuming basic research, which estab-

lished the top-level concepts and the format of each type of

entry, a major effort was devoted to the development of

semi-automatic tools of acquisition enabling the participation

in the work, at its simplest level, of rank-and-file enterers

with a minimal training. This was achieved by an inventive

use of easy-to-understand templates for several basic types

of concepts. The introduction of the tools made the acqui-

sition of numerous concepts by minimally trained workers

very speedy and inexpensive.

The resulting ontology meets and exceeds the industrysstandards of clarity, coherence, and extendibility, and it

stands to reason to conclude that any ontology of the future,

comparable to this particular one in size and depth, will have

to be developed more or less along the same lines: a combi-

nation of the tiered approach to acquisition, the use of semi-

automatic tools, and the emphasis on cheap labor whenever

and wherever practical.

2.3 ONTOLOGY AND NATURAL LANGUAGE

It is in the development of MT and other natural language

processing (NLP) systems that the ontological approachproved to be particularly successful (see Nirenburg and

Raskin 2004). Early MT was of the transfer type: a system

was designed on the basis of a minimally required rules of

transfer from words of the source language into words of the

target language. While the simplest transfer, word for word,

does not work in translation, inventive rules were created to

overcome this difficulty in many cases. The resulting trans-

lation was, however, pretty rough and not very accurate,

making it hard to use without expensive human post-editing.

The alternative to transfer MT was the interlingua

approach. Interlingua was a formal representation of text.

The input text was analyzed and represented as an inter-

lingua text and the latter was used to generate an equivalent

text in the target language. An ontology was introduced as

a rich and sophisticated form of interlingua in the late

1980s and won a dominant position in the interlingua

approach in the 1990s.

The MikroKosmos ontology is the foundation of the

whole knowledge- and meaning-based approach to lan-

guage processing. As a universal language-independent

resource, the ontology is the metalanguage for the lexicon:

each lexical entry in a natural language is defined in onto-

logical terms. In the simplest case, the meaning of a word

corresponds exactly to an ontological concept; in this case,

the entry contains a simple pointer. More often, the match isnot exact and an additional constraint, formulated in terms

of ontological properties, must be added to the entry. Yet in

other cases, the entry must contain a reference not to a

concept but rather to a certain slot and/or filler in its frame.

Even more importantly, the ontology provides a

conceptual foundation and world grounding for whole texts.

Working with the lexical entries making up a sentence and

using the pertinent syntactic information for the natural lan-

guage being processed, the analyzer produces ontology-based

formula, the text meaning representation, for the sentence.

158 Ontology