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

10 Chapter 2. Background Knowledge
2.1 Lexical Semantics
In theoretical linguistics, morphology deals with the identification, analysis and
description of the structure of words, syntax deals with the study of structural
relationships between words in a sentence, and semantics studies the meaning
of language. In order to interpret words, phrases, sentences and texts, natural
language is mapped to a formal language. Formal models of semantics are suitable,
for instance, for being handled by machines.
Our research is especially focused on semantics, more precisely on the meaning
of words. Lexical semantics is the subfield of semantics that studies the words of
a language and their meanings, and can be seen as the bridge between a language
and the knowledge expressed in that language (Sowa, 1999). Lexical semantics has
been the subject of studies since the early nineteenth century, so it is no surprise
that several theories on this topic have been developed (see Geeraerts (2010) for an
extensive overview on theories of lexical semantics).
2.1.1 Relational Approaches
Among the theories of lexical semantics, the relational approaches are probably
the most widespread (Geeraerts, 2010). They describe the meaning of words and
expressions (hereafter, lexical items) through relationships. Some authors call
them lexical relations (Cruse, 1986), others semantic relations (Murphy, 2003),
and others sense relations (Jurafsky and Martin, 2009; Geeraerts, 2010), because
these relationships are held between meanings of the lexical items, often called
senses. In this thesis, we have adopted the term semantic relation.
Dictionaries are probably the main human-readable source of information on
vocabulary and meaning of a language, as they are organised repositories of lexical
items and information about their possible senses. Dictionaries are created by
lexicographers, experts in the description of meanings in natural language. Sense
definitions are often explained with reference to related lexical items (e.g. category,
parts, synonyms), which (implicitly) indicate one or more semantic relations.
Nevertheless, even though dictionaries may be seen as a rough approximation
to the way humans structure their knowledge about language, they are different
from what is usually referred to as the mental lexicon. To begin with, in a dictionary,
it is impossible to list extensively all the possible meanings of a lexical item,
as the potential uses of a word are limitless (Nunberg, 1978). The mental lexicon
is structured on concepts, which may be represented by words in the process of
lexicalisation, but are defined by much more arbitrary rules, depending on each
others’ experiences. As word senses are not discrete and cannot be separated with
clear boundaries, sense division in dictionaries is artificial (Kilgarriff, 1996; Hirst,
2004). Furthermore, while general language dictionaries contain mainly knowledge
about the language itself, in the mental lexicon it is hard to make a clear distinction
between the latter and knowledge about the world, commonly referred to as
encyclopedic knowledge.
In order to increase the utility of the mental lexicon as an object of study and as
a valuable resource, some theoretical approaches, and most computational, adopted
a more structured and practical representation for this idea, where several simplifications
are made. At the same time, these models make explicit some information