Another Look at Nominal Compounds
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ANOTHER LOOK AT NOMINAL COMPOUNDS Pierre Isabelle D~partement de linguistique Universit~ de Montreal C.P. 6128, Succ. A, Montreal, Qua., Canada H)C )37 ABSTRACT semantics. And since noun meaning appears to be closely connected with knowledge of the world, one is led to explore the modes of interaction We present a progress report on our research between linguistic and conceptual knowledge. on nominal compounds (NC's). Recent approaches to this probiem in linguistics and natural ianguage From an NLP perspective, NC's have turned processing (NLP) are reviewed and criticized. We out to be an important stumbling block for sys- argue that the notion of "roie nominal", which is tems that attempt to deaI with real-iife text, at the interface of linguistic and extraiinguis- especiaIly in technical domains, for purposes tic knowledge, is crucial for characterizing NC'e such as machine translation (IsabeIle, to as weII as other Iinguistic phenomena. We examine appear), information retrieval, etc. a number of constraints on the semantic interpre- tation ruies for NC's. Proposals are made that Our ultimate goal is to develop an NLP sys- shouid improve the capability of NLP systems to tem capable of analyzing large classes of NC's in deaI with NC's. the sublanguage (Kittredge and Lehrberger, i982) of aircraft maintenance manuals. We do not aim at solving all cases, since we believe the problem to be exceedingly difficult. At the present stage I INTRODUCTION of our inquiry, we concentrate on the design of s suitable theoretical framework. A. Problem Statement In section II, we present a brief review of As a first approximation, we define a previous work in linguistics and NLP. In sections "nominal compound (NC) as s string of two or more III and IV, we examine two aspects of the seman- nouns having the same distribution as a singie tics of nouns that are crucially relevant to the noun, as in example (I): analysis of NC's: predicative nouns and role nominals. Finally, in section V, we explore pos- (1) aircraft bomb bay door actuating cylinders sible constraints on the semantic interpretation of NC's. We will see below that provisions have to be made in some cases for intervening adjectives. NC's can exhibit various degrees of lexica- II BACKGROUND lization, but we will focus our attention on productive rules for forming novel compounds. As to their surface syntax, NC's can assume any A. Approaches in Linguistics structure generated by the rule N --> N N; accor- dingly, their structural ambiguity grows exponen- The early study of Lees (1963) classified tially with their length (following the "Catalan NC's on purely grammatical criteria, and it sequence"). How, then, do we determine that the failed to provide constraints that could explain normal interpretation for (i) imposes the bracke- how NC's are semantically interpreted. ting shown in (2), rather than any of the other 41 syntactically possible bracketings? In s number of more recent studies, such as Levi (1978)9 there has been an attempt to view (2) ((aircraft ((bomb bay) door)) NC's as governed by tight semantic constraints. (actuating cylinder)) Thus, according to Levi, any novel NC realizes a pattern where either: a) the head noun is a deverbal naminalizstion and its modifier is interpreted as an argument of the related verb; B. Goal__ssof th___ee study or b) the two nouns are related by one of exactly nine deletable predicates ("is the cause of", "is We believe that the analysis of NC's repre- for", etc.). sents an important and largely unsolved problem. From a theoretical point of view this problem This reductionist attempt has been criti- raises the question of how to deal with noun cized, most notably by Downing (1977), on the 509 grounds that the interpretation of NC's crucially (3) a. measure nouns involves pragmatic knowledge, and that numerous map: objects onto quantities cases of NC's (such as thalidomide parents) will examples: speed (of), temperature (of), resist any analysis in terms of a closed set of volume (of), size (of) relations. These criticisms have led theorists like Selkirk (1982) to adopt the position that b. "area" nouns only "verbal compounds" (those constructed on a map: objects onto subparts pattern "argument + nominalization") are amenable examples: top (of), side (of), bottom to linguistic characterization; all other NC's (of), center (of), core (of) would have to be explained in separate extralin- guistic theories. c. collective nouns map: individuals onto sets examples: group (of), set (of) C. Approaches in NLP d. representational nouns Several systems have been developed in an map: objects onto representations of ob- NLP framework to deal with the problem of inter- jects preting NC's; two recent exempIes are reported in examples: picture (of), diagram (of), Finin (1980) and McDonaId (i982). sense (of) In both systems, the individual nouns of an e. other examples NC are first mapped onto conceptual representa- location (of), goal (of), tions where concepts are characterized by a set brother (of), king (of) of "roles" or "slots", and are arranged in an abstraction hierarchy. Interpreting a compound Most if not all of these argument-taking nouns then amounts to forming a derived concept on the can have their argument satisfied by a modifier basis of the constituent concepts; in most cases, noun in an NC: this is done by interpreting one of the concepts as a slot filler for the other. For example, the (a) a. oil temperature interpretation of steel adapter would involve b. box top inserting the concept associated with steel into c. tank group a RAW-MATERIAL slot within the representation of d. circuit diagram adapter. But the authors do not examine in any e. component location detail the question of eventual constraints on this interpretation process, such as the effect A treatment in terms of predicate/argument pat- of word order. terns for this type of NC seems far superior to Levi's (1978) use of the semantically empty Another crucial issue which has not been "deletable predicate" HAVE ("the component HAS a explored in sufficient detail, is the nature of, location"). Although these NC's are excluded from and the justification for the particular set of Selkirk's (1982) class of verbal compounds, they slots which is assigned to a given concept. Finin are amenable to the same type of semantic des- is somewhat more explicit on this question. Some cription. nouns have slots which represent standard case roles. Role nominals, on the other hand, are nouns which refer to a particular case role of B. Action and State Nominalizations another concept; for example, food refers to the object role of eat, and this fact provides the Most studies on NC's have recognized the key to the interpretation of cat food. This fact that deverbal nominalizations exhibit a notion will be examined in detail below. But semantic behavior closely related to that of the Finin also resorts to other types of slots (e.g. verb, and subcategorize elements that can occur "raw-material") which are not discussed. as modifier nouns in NC's. As mentioned above, these are in fact the only cases that Selkirk (1982) deems characterizable at the level of linguistic competence. III PREDICATIVE NOUNS 8ut there seems to be no reason why deadjec- tival nominalizations should be handled in a different way. In examples (5) and (6), an action A. Root Nouns with Arguments and a state are nominalized, with the argument occurring either in a prepositional phrase or as The fact that several classes of non-derived a modifier in an NC: nouns strictly subcategorize phrases which are semantically interpreted as arguments has (5) a. Someone removes the pump. received very little attention in the literature. b. removal of the pump This phenomenon would deserve an extensive study, c. pump removal and we can only give here some relevant examples, together with an indication of the semantic cate- (6) a. Uranium is scarce. gories between which the relation expressed by b. scarcity of uranium the noun effects a mapping: c. uranium scarcity 510 We are not claiming that there is exact A related claim underlies Zholkhovskij & synonymy betweenthe (b) and (c) examples, but Hel'cuk's (1971) use of certain "lexical func- only that they exhibit the same predicate/ tions": argument pattern. Action nominals can take various types of arguments in NC's, and sometimes (15) a. Sl(bUy) = buyer several of them simultaneously: b. S3(buy) = seller c. Sloc(battle) = battlefield (7) a. pump failure (subject) d. Sinst(See) = eyes b. Montreal flight (source, goal) (8) a. poppet chatter tendency where Si, $1o c and Sinst are defined as functions t~t yield the typical name of, respectively, the i- arrant, the location a-nd the instrument. IV ROLE NOMINALS Fillmore (1971) also makes a comparable proposal, when he suggests that the lexical entry of knife should include (16) as a component: A. Nominalizations (16) use: I of <V 0 1 A> Deverbal nominalizations can refer not only where V=cut to the action expressed by the verb, but also to the agent (driver), instrument (lubricant), pa- tient (employ-~-~and result (assembly) of this action. Except maybe for results, the term "role C. On the Definition of Role Nominals nominal" seems appropriate, since the nominaliza- tion refers to the filler of one of the roles of Mow exactly should we understand the notion the verb. Although these nouns are not, strictly of role nominals? Finin's statement that they speaking, predicative, they generally permit to refer to an underlying case role of a verb is not form NC's in which the other is interpreted as an accurate: they refer to role fillers, not to the argument of the underlying verb: roles themselves.