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<Paper uid="W98-1215">
  <Title>Sense Variation and Lexical Semantics Generative Operations</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Investigations within the generative perspective aim at modelling, by means of a small number of rules, principles and constraints, linguistic phenomena (either morphological, syntactic or semantic) at a high level of abstraction, level which seems to be appropriate for research on multi-linguism and language learning. These works, among other things, attempt at modelling a certain form of 'creativity' in language: from a limited number of linguistic resources, a potentially infinite set of surface forms can be generated. null Among works within the generative perspective, let us concentrate on the Generative Lexicon (Pustejovsky 91, 95), which has Settled in the past years one of the most innovative perspective in lexical semantics. This approach introduces an abstract model radically opposed to 'flat' sense enumeration lexicons. This approach, which is now well-known, is based (1) on the close cooperation of three lexica\] semantic structures: the argument structure (including selectional restrictions), the aspectual structure and the Qualia structure, (2) on a detailed type theory and a type coercion procedure and (3) on a refined theory of compositionality. The Generative Lexicon (GL) investigates the problem of the multiplicity of usages of a sense of a lexeme and shows how these usages can be analyzed in terms of possible type shirtings w.r.t, the type expected by a usage of that sense defined as the core usage. Type shifting is modelled by a specific inference mechanism: type coercion. The GL shows very clearly the inter-dependence between arguments and predicates.</Paragraph>
    <Paragraph position="1"> In our perspective, we are not only interested in deciding whether an expression is an acceptable argument for a predicate and for what reasons as it is also the case in the GL, but we want to be able to 'reconstruct' or infer the meaning of the proposition from its parts (the predicate and its arguments), and possibly also from the implicit semantics conveyed by the syntactic form (Goldberg 94). We assume that the impossibility of building a semantic representation for a proposition entails that it is semantically ill-formed w.r.t, our grammar, lexicon and composition rules. The work presented here is not definitive, it is a feasability study which aims at opening new perspectives and new treatments for the GL, while remaining in the same spirit. We will in particular show that in some cases, the Qualia structure can be combined with, or replaced by, a set of relatively general rules, which, in addition, better capture the relations between the predicator and the predicated elements.</Paragraph>
    <Paragraph position="2"> In this paper, we contrast a rule-based approach (also used by other authors such as (Copestake and Briscoe 95), (Ostler and Atkins 92), (Numberg and Zaenen 79) with different perspectives) with the Qualia-based approach and comment on their respective advantages. We show how, in fact, they can cooperate. Another view is that presented in (Jackendoff 97, chapter 2) with his principle of enriched composition, which is in fact quite close to our view, but restricted to a few coercion situations (aspectual, mass-count, picture, begin-enjoy). As will be seen, these systems are not incompatible, they cover different forms of knowledge and may be useful in different situations. The rules we present here are not lexical rules, as in (Copestake and Biscoe 95), but they are part of the semantic composition sys-Saint-Dizier 121 Sense Variation and Lexical Semantics Patrick Saint-Dizier (1998) Sense Variation and Lexicai Semantics Generative Operations. In D.M.W. Powers (ed.) NeMLaP3/CoNLL98: New Methods in Language Processing and Computational Natural Language Learning, ACL, pp 121-130. tern. Compared to the Qualia system (Pustejovsky 91, 95), the different senses which can be derived from a more central sense are constructed in our system in context i.e. when dealing with arguments, not a priori as in the Qualia roles. Finally, in the last section of this document, we show that some problems advocated by (Copestake and Briscoe 95) concerning unification can be resolved by a constraint-based approach.</Paragraph>
    <Paragraph position="3"> Before going on into more details about our approach, here are some general comments about lexical semantics generative systems.</Paragraph>
    <Paragraph position="4"> In a lexical semantics generative system, it is of much importance to have a clear analysis of the notions of word-sense and of sense delimitation. Indeed, depending on the strategy adopted (e.g. narrow senses as in WordNet, or very large ones as in many AI works), the nature and the scope of generative operations, and therefore the philosophy of language behind these operations, may entail very different sense delimitation strategies. We do not think that it is possible to define a real theory of sense delimitation, but it is certainly possible to define a few principles or a strategy. Then, given a strategy for sense delimitation, we can adjust the complexity and the scope of generative operations.</Paragraph>
    <Paragraph position="5"> We assume that a verb potentially contains a number of subsenses which are 'triggered' by semantic composition rules depending on the arguments found in a sentence. There are several possibilities to represent a sense and its relations with subsenses, our approach is, as shall be seen below, to develop under-specified Lexical Conceptual Structure (LCS) (Jackendoff 91) representations, that will also be used as a basis for developping generative mechanisms, It should be noted that, apart from fixed forms, sub-senses share quite a lot of features and are often distinguished by a few features.</Paragraph>
    <Paragraph position="6"> As shown in (Pustejovsky 95) and (Copestake and Briscoe 95), type coercion does not occur within the verb, but at the level of the proposition, allowing a sentence such as: Mary enjoys the film and eating ice-cream.</Paragraph>
    <Paragraph position="7"> This type of sentence is problematic for unification because the verb must bear two types for the first object, syntactically an NP and a proposition (Sbar) and semantically a physical object and an event.</Paragraph>
    <Paragraph position="8"> A constraint-based approach which allows domains to be assigned to variables (the variable representing the semantics/syntax of the object) would be a good solution (see section 6).</Paragraph>
    <Paragraph position="9"> In spite of evidence of its existence (explored in the Euronet project, the European WordNet), the Qualia structure turns out to be quite difficult to describe. It is well-designed for nouns, and looks more artificial for other lexical categories, but it is in fact essentially useful for predicated elements, mostly nouns. In fact, it seems that Qualia structures are not really motivated and relevant for predicators. This can be noted in a concrete way when looking at verb Qualias which do not turn out to be very satisfactory and expressive, but rather look a little bit artificial. The descriptions made in roles are also often more pragmatic than semantic, and this may entail additional confusions in resolving type mismatches. This is in fact clearly stated in various analysis of the Aristotelician AITIAs, from which the Qualia roles emerged (M0ravcsik 75), where the author shows that there is a kind of continuum between telic and agentive roles. Finally, the formal and constitutive roles are also somewhat redundant with the part-of and isa relations used to structure lexicons (or similar relations used in thesauri), it may not therefore be totally necessary to include them in the Qualia.</Paragraph>
    <Paragraph position="10"> A close observation of linguistic data and of sense variations shows that the most important role is the telic role. It is in fact the role to be considered as a default role for the coercion object -+ event. The formal role is far less frequently considered. We also think that this role should not only describe the coming into being of entities but also their destruction, which cannot enter into the telic role.</Paragraph>
    <Paragraph position="11"> We do not thing that generativity can only be based on an elaborated theory of types, and on the operation of type coercion. This operation is very powerful, simple and works satisfactorily for a number of cases, but it turns out in some cases to be insufficient and also to largely overgenerate (in particular it is quite d i~cult to use in natural language generation systems), unless a very rich and refined theory of type is defined, which may be contrary to the type philosophy. We think that the type system is well-designed to detect derived usages, characterizable by a type mismatch. The generality and genericity of the type coercion operation should however be kept because, via the reference to quite general types, it preserves a good degree of generality and systematicity to type shiftings. However, we feel that types are not sufficiently 'constrained' to account for the constraints holding, for each verb, on the different sense/usage variations it may be sub-ject to. For example, an underspecified LCS form could be more appropriate because of its ability to partially represent meaning. The Qualia structure cannot encode all these constraints, in particular first because they are better designed for arguments than for predicators and second because some con-</Paragraph>
    <Paragraph position="13"> straints are relational between the predicator and the argument, which the roles cannot describe.</Paragraph>
    <Paragraph position="14"> In this document, we first present a standard classification of the different forms of relations between a predicate and its arguments, then we outline some considerations on the difficult problem of sense charaterization and delimitation. We then show how it is possible to give up the principle of a Qualia structure and, in some cases, of type coercion in favor of (1) other sources of knowledge, essentially inherent properties of lexical items and (2) simple inference rules. This approach may be felt to somewhat resemble meaning postulates, but it is organized on a larger scale and treats different phenomena. null</Paragraph>
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