Onto.PT: Towards the Automatic Construction of a Lexical Ontology ...

14 Chapter 2. Background Knowledge
2.2 Lexical Knowledge Formalisms and Resources
Language dictionaries are probably the primary source of lexical semantic knowledge
of a language, and are of great utility for humans to acquire information on
words, their possible uses and meanings. However, they are not ready for being
used as computational lexical-semantic resources, because they do not contain explicit
knowledge on the meaning of words. Figure 2.1 is an example of an entry of
the Longman Dictionary of Contemporary English (LDOCE) 5 .
dictionary, noun
dic-tion-a-ry, plural dictionaries [countable]
1. book that gives a list of words in alphabetical order and explains their meanings in the
same language, or another language:
[usage] a German - English dictionary
2. a book that explains the words and phrases used in a particular subject:
[usage] a science dictionary
Figure 2.1: Entry for the word dictionary in the LDOCE.
Dictionaries are structured on word senses, and typically contain other kinds of
information on lexical items, as their etymologies, part-of-speech, syllabic division,
domain and usage examples. But they describe senses in natural language, which
may sometimes be vague or ambiguous. Therefore, in order to be used by computational
applications, they need to be parsed and the lexical-semantic knowledge in
their definitions needs to be formalised.
In the rest of this section, we will introduce three basic formalisms for representing
meaning. Then, we will describe some of the most common structures used
in the development of computational lexical-semantic resources, typically known as
lexical knowledge bases (LKBs).
2.2.1 Representation of Meaning
In opposition to dictionaries, there are formal representations for the meaning
of words and sentences, following the relational approach to lexical semantics. The
most popular abstractions for representing meaning in a formal language are logical
predicates (Smullyan, 1995), directed graphs (Harary et al., 1965) and semantic
frames (Fillmore, 1982). These formalisms are in many ways equivalent and can
often be converted to one another. Figures 2.2, 2.3 and 2.4 illustrate, respectively,
the three formalisms, by representing the meaning of the following sentences:
The bottle contains wine. Wine is a beverage.
The basic formalisms for representing meaning can be combined, reorganised
and evolve into other, eventually more complex, structures, for representing the
meaning of a (natural) language, in an abstract model of the mental lexicon. When
materialised into LKBs, including thesauri, taxonomies and lexical ontologies,
5 LDOCE is available for online queries from http://www.ldoceonline.com/ (September 2012)