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

78 Chapter 4. Acquisition of Semantic Relations
where transitivity was applied to the synonymy relations of PAPEL, giving rise to
some inconsistencies as the following:
• queda synonym-of ruína ∧ queda synonym-of habilidade
→ ruína synonym-of habilidade
The problem occurs because one sense of queda is the result of falling, while
another means to have some skill. Therefore, combining those two, we obtain that
ruína (ruin) is the same as habilidade (ability, skill), which are almost opposites.
Nevertheless, since the beginning of the project PAPEL, our option was to build
a lexical resource where lexical items were not divided into word senses. That early
option relied on the following:
• From a linguistic point of view, word senses are not discrete and cannot be
separated with clear boundaries (Kilgarriff, 1996; Hirst, 2004). Sense division
in dictionaries and lexical ontologies is most of the times artificial.
• Following the previous point, the sense granularity in dictionaries and lexical
ontologies is often different from lexicographer to lexicographer. As there is
not a well-defined criteria for the division of meanings, word senses in different
resources do not always match (Dolan, 1994; Peters et al., 1998).
• Word sense disambiguation (WSD, see Navigli (2009b) for a survey) is the task
of, given the context where a word occurs, selecting the most adequate of its
senses from a sense inventory. However, the previous points confirm that WSD
is an ill-defined task and is very dependent on the purpose (Wilks, 2000).
• Dictionaries do not provide the sense corresponding to a word occurring in a
definition. After the first version of PAPEL was released, Navigli (2009a) actually
presented a method for disambiguating words in dictionary definitions.
Still, given the aforementioned problems on WSD, the term-based structure
of PAPEL was kept.
• Finally, in natural language, the study of vagueness is as, or even more, important
that studying ambiguity (see e.g. Santos (1997)).
When we started to extract relations from other dictionaries (and thesauri), we
confirmed that the senses of words occurring in more than one resource did not match
for different resources. Moreover, not all definitions in Wiktionary.PT have a sense
number and synonymy lists do not always indicate the corresponding synonymous
sense. Since we are extracting information from more than one lexical resource, an
alternative would be to align the word senses in different resources (represented as
definitions in dictionaries or synsets in thesauri), as others did (e.g. Vossen et al.
(2008); Henrich et al. (2012)). Still, given the aforementioned utility of a lexical
resource as PAPEL, we decided to keep CARTÃO as a term-based resource.
In the following chapters, we explain how the structure of CARTÃO can evolve
to a resource that handles word senses. After the additional steps of the ECO
approach, the result is Onto.PT, a resource structured in synsets. We recall that
this approach is flexible in a way that it enables the construction (and further
augmentation) of a wordnet, based on the integration of knowledge from multiple
heterogeneous sources and, from this point, it does not require an additional analysis
of the extraction context. The only requirement is that the initial information is
represented as tb-triples, which is kind of a standard representation.