Pustejovsky (1991, pustejovsky:95a) has argued that there are certain core lexical semantic elements of a nominal which must be represented in its lexical entry in order to achieve a more generative description of lexical sense derivation. Pustejovsky proposes that these core elements can be captured by several roles in a structure called qualia structure, based on Aristotle's notion of modes of explanation which drive our basic understanding of an object or a relation in the world, which specify four essential aspects of a word's meaning:
The two components of qualia structure which are most directly relevant to the phenomena in this thesis are the TELIC and AGENTIVE roles. I will interpret the TELIC role as indicating a particular event involving the artifact which most directly expresses the use to which the artifact is stereotypically/conventionally put. For example, liquids which are typically ingested by humans, such as milk, beer, and water will be specified for a drinking event in their TELIC role. The value of this role will be formalised as a semantic relation from the sort hierarchy, for which the argument role played by the denotation of the noun in question is indicated via structure sharing with the noun's index. The AGENTIVE role will also be filled by a semantic relation from the sort hierarchy, this time indicating an event which caused the creation of the denotation of the noun. For a cake, for instance, this role might be given the value of bake-rel and the index of the noun will be associated with the und argument of this relation.
Nouns will also be added to the ontology captured in the sort hierarchy, in order to capture relations between entities and to make use of inheritance to efficiently characterise related semantic types. In the hierarchy for nominal objects I will follow copestake:92 in attempting to capture general information that is true about individual nominal types. As a result, much of the information associated with the nominal types will be inherited by default -- specific instances of a nominal type may not be accurately characterised in terms of the information it receives by default through the specifications in the hierarchy. However, the representation of this information should allow verbs to specify selectional restrictions to a certain extent, and to account for co-compositional behaviour.
In order to allow for defaults and default inheritance, I assume an order independent default unification for typed feature structures as defined by Lascarides et al (1996). Typed feature structures, as defined by the sort hierarchy, can include default information (which is marked as being default) which can be inherited by their subsorts or overridden with sort-specific values. An example of the application of this default inheritance to TELIC role values from Lascarides et al (1996) and adapted to the representation formalism introduced here, is shown in Figure 2.15.
Figure 2.15: Inheritance of the Telic roles of artifacts (Lascarides
et al 1996)
Defeasibility is indicated with a slash notation: values are of the form indefeasibledefeasible, which is abbreviated in the figure to /defeasible where the indefeasible value is completely general. An operation called Persistent Default Unification (PDU) is defined to capture the default inheritance appropriately.
Figure 2.15 reflects several features of the representation of nominal semantics I assume.