Collative Semantics

This paper introduces Collativc Semantics (CS), a new domain-independent semantics for n~tural language processing (NLP) which addresses the problems of lexieal ambiguity, met(> nymy, various semantic relations (conventional relations, redundant relations, contradictory relations, metaphorical relations and severely anomalous relations) aud the introduction of new information. We explain the two techniques CS uses for matching together knowledge s t ructures (KSs) and why semantic vectors, which record the results of such matches, arc informative enough to tell apart semantic relations and be tile basis for lcxical disa.mbiguation. 1. I n t r o d u c t i o n Collative Semantics is a new domain-independent sem,~ntics for NI,P. This paper foeusses on CS, describing tile main issues it addresses (lexical ambiguity, mctonymy, semantic relations, introduction of new information) and general details of its knowledge representation, knowledge structures, techniques for matching together knowledge structures, and the way it records the results of matching in semantic vectors. CS has l)een implemented in a natural language program eaiicd recta5 which has been described in detail clsewherc (Fass, 1986). Briefly, the program produces semantic representations of single sentences requiring lexical disambiguation and met(> nymic infereneing. While processing such sentences recta5 computes semantic vectors for the matches between pairs of wordsenses which are rich enough to discriminate between instances of conventional relations, semantically redundant relations, contradictory relations, metaphoricM relations and severely anomalous relations between word-sense pairs. Semantic vectors also record the assertion of new information. Meta5 can process all the example sentences given in the next section. 2. S e m a n t i c R e l a t i o n s , N o v e l I n f o r m a t i o n a n d M e t o n y m y In this section we provide brief descriptions of those semantic phcnomcna considered by CS, except lexical ambiguity, s tar t ing with semantic relations. Conventional, metaphorical and severely anomalous relations can all be described using the terminology of Richards (1936). The subject term is the "topic," the term it is compared to is the "vehicle," the similarity or resemblance between them is the "ground," and any difference or dissimilarity is tile "tension." We also adopt Perrine's (1971) four-fold classification of metaphors into combinations of explicit and implicit topics and vehicles. In a metaphorical relation there is tension between the topic and vehicle because the topic term is not a type of vehicle term. Wha t is salient (Ortony, 1979) is given by the context, a salient property of the vehicle is found and an analogical match discovered between it and u property from the topic. The remaining properties of the topic and vehicle have similarities and differences and, although these are not central to recognising the metaphorical relation, the higher the proportion of differences to similarities the "bet ter" the metaphor. For example, in the metaphorical relation between 'cal" and 'dr ink ' ill (1) "The car drank gasoline." the topic is 'ear ' and the implicit vehicle is 'animal ' , the agent preference Wilks, 1975) of 'drink'. The tension is caused by a car (the topic) not being a type of animal (the vehicle). Wha t is salient in tiffs context is the action of drinking, given by the main sentence verb. The salient property of the vehicle is one referring to the salient action, i.e. tha t an animal dl'inks potable liquids. An analogical match is found between tile salient property of the vehicle and a property of the topic : animals drink potable liquids as cars use gasoline. The ground is the expending of liquid. Matching the remaining properties of the topic and vehicle, some pairs of properties are the same (animals and ears are both bounded, three-dimensional and solid) but other pairs express differences (animals are living, cars are nonliving; animals are composed of flesh, cars are made of metal). Ill a conventional rclation there is no tension because the topic term is a type of vehicle term. The sMient property of tile vehicle matches identically with a property from tile topic.. There is a high proportion of similarities to differences amongst matches of other pairs of properties of the topic and vehicle. For example, ill the conventional relation between 'man ' and 'drink' in (2) "The mall drank beer." the topic is 'man ' and the implicit vehicle is again 'animal ' , the preferred agent of 'drink' . A man (the topic) is type of animal (the vehicle). Wha t is salient is drinking so the salient property of animals is again tha t they drink potable liquids. An identical match is found between animals drinking potable liquids and men drinking potable liquids. Of the remaining properties of animal and man matched together, a large proportion are similar and very few are different. A severely anomalon~'~ relation has the same tension between topic and vehicle a~ a metaphorical relation : the topic term is not a type of vehicle term. A salient property is found in the vehicle but it does not find an identical or analogical. match with any property from the topic. A high proportion of the remaining properties of the topic and vehicle matched together are different and few are the same. Our description of semantically redundant and contradictory relations is based on Katz 's (1964) definitions. Whether the assertion of a particular property onto a subject term is a redundant relation, contradictory relation or new information is a function of tile knowledge already in the term's dictionary entry. A contradictory relation is one in which a modifier asserts a property onto a subject term which is incompatible with a property already in the subject term, e.g. (3) "John McEnroe is female." where the assertion [sexl, femalel I from the adjective clashes