Abductive Reasoning For Syntactic Realization

Abductive reasoning is used in a bidirectional framework for syntactic realization and semantic interpretation. The use of the framework is illustrated in a case study of sentence generation, where different syntactic forms are generated depending on the status of discourse information. Examples are given involving three different syntactic constructions in German root clauses. 1 Pragmatics in Natural Language Generation The computational treatment of pragmatics in natural language generation is often—directly or indirectly— oriented around the Gricean maxims [Grice 75]. Their effects emerge from the pragmatic model of the generation system so that the generated texts satisfy these maxims. The texts should be a true characterization of a state of affairs, they should be as informative as possible, relevant, and perspicuous. While the first three maxims are related to what is said, the last maxim is related to how it is said. The category of perspicuity principles includes constraints on avoiding obscurity and ambiguity, or being brief and orderly. It is anything but clear how these principles should be interpreted precisely. Several attempts have been made to remedy this in computational work on generating texts that best satisfy these maxims, especially with respect to the generation of referring expressions (e.g. [Dale et al. 95]). However, there is more to pragmatics than satisfying Gricean maxims. In particular, the category of perspicuity principles does not usually cover the important fact that texts are tailored to a specific addressee, not only in content, i.e., with respect to her or his informational needs, but also in the linguistic form, i.e., word order, syntactic constructions, the choice of lexical items, and eventually prosodic information. This tailoring of the linguistic form to the listener is termed “information structuring”. In generating texts, information structuring requires, among other things, the use of some listener model, which may include information about the listener’s knowledge, goals, properties, etc. Linguistic approaches to describing the principles of information structuring have sometimes characterized information structure as an instruction to the listener about how to construct a model of the communicated state of affairs [Prince 81]. In AI and Computational Linguistics, tailoring the message to the user comprises very often solely content planning, which only indirectly determines the linguistic output. For example, systems tailor the information “density” to the user (e.g. [Paris 93]), or they drive the dialogue depending on an estimation of what the user might be interested in (e.g. [Jameson et al. 94]). Realizing texts by determining the information structure of the respective sentences, which again is a reflex of addressee orientation, has not yet received its due attention. ∗The authors would like to thank Bob Kasper, Nathan Vaillette, Shravan Vasishth, and two anonymous referees for helpful comments and suggestions. All remaining mistakes are, of course, our own. 2 The Topic/Comment Structure in Information Structuring The notion of information structure comprises at least two separate notions of how the information of a sentence may be structured, viz. the topic/comment structure and the focus/background structure [Vallduvı́ 92, Lambrecht 94].1 In order to motivate these structuring mechanisms, consider the following simple example. Suppose the purpose of a generation system is to describe a spatial scenario. One of the sentences might be (1) Behind the town hall is a BAKERY. with “bakery” the prosodically most prominent constituent (the focus exponent). In this sentence, the prepositional phrase “behind the town hall” functions as topic and the noun phrase “a bakery” is in focus. we will ignore aspects of focus and its role in language generation, especially since selecting the focus exponent is better understood as being part of utterance planning, and are limiting our attention to the topic/commentstructure only. The topic provides familiar discourse referents whose properties are further illuminated by the sentence; the relation between these discourse referents and the sentential predication is also referred to as an aboutness-relation. Many languages possess special topicalization constructions or morphological markers to single out the topic in a sentence. In German (and probably English as well), referring phrases provide topic referents, and the clause-initial position is their preferred position. Thus clause-initial positioning is the most important topic-relevant feature in generation.2 The same propositional content expressed by (1) can be realized with different information structures and, therefore, different sentence forms, as the following English examples demonstrate: (2) A bakery is behind the town hall. (3) Behind the town hall, there is a bakery. (4) As for the town hall, behind it is a bakery. Discourse referents functioning as topics must be identifiable for the listener. This is the reason why topics are usually packaged as definite noun phrases, or as prepositional phrases that contain definite noun phrases. Topic candidates will be selected from the set of discourse referents that the listener knows according to a topic acceptance scale. [Lambrecht 94] proposes the following scale: (5) active > accessible (textually, situationally, or inferentially) > unused > brand-new anchored > brand-new unanchored Active referents are those that are currently lit up; they are in the center of attention. They are the most acceptable topics because the listener’s mental effort needed for processing the respective sentence is minimal as compared with the effort needed to identify and anchor an unfamiliar or inactive topic referent. We consider the candidates below the accessible referents to be inappropriate as topics in most instances, and we limit our attention to a scale with three regions: active referents, accessible referents, and inaccessible referents. To summarize, the first task in generating texts with sentences with appropriate topic/comment structures is to determine for each sentence the topic discourse referent. This referent should be identifiable for the listener and as high on the topic acceptance scale as possible. The phrase expressing the topic should 1Depending on one’s theoretical background and/or affiliation with different schools, terminology differs considerably. 2[Altmann 81, 150] gives some counterexamples to this default. These examples are prosodically marked, however. be placed in clause-initial position. However, these are only guidelines, not fixed rules. Hence, we need a mechanism to handle this kind of uncertainty. This is of course not the whole story of topic-hood. In addition to selecting topic referents, we have to solve the problem of how one and the same topic/comment structure can be realized by different syntactic structures. German examples resembling the previous three ones are: (6) Die Vitrine steht rechts von der Lampe. ‘The showcase is standing to the right of the lamp.’ (subject realization) (7) Die Vitrine, die steht rechts von der Lampe. ‘The showcase, it is standing to the right of the lamp.’ (left dislocation) (8) Was die Vitrine betrifft, die steht rechts von der Lampe. ‘As for the showcase, it is standing to the right of the lamp.’ (hanging topic) In the first clause the topic is realized as the subject in clause-initial position. The second clause exhibits a left dislocation for the topic, and the third one uses a so-called hanging topic. We assume that the functions of these three syntactic forms are more or less identical for German and English. All three examples express the same propositional content, viz. the localization of a uniquely identifiable showcase with respect to a uniquely identifiable lamp. Furthermore, all three examples exhibit the same topic/comment structure: “the showcase” functions as topic, i.e., the anchor for the proposition, and the rest of the clause comments on certain aspects of the showcase. However, these three forms are not mutually interchangeable in each imaginable context, because they invite different pragmatic inferences. The subject realization is neutral with respect to topic accessibility. There is a strong correlation between the grammatical function of subject and the information structural notion of topic. The subject is the unmarked topic. Left dislocation constructions, they can indicate a topic shift because the syntactically autonomous position of the detached noun phrase signals a change in the status of its discourse referent from being inactive to active [Lambrecht 94]. Additionally, left dislocations must satisfy a presupposition condition, namely to support the existence of another individual not having the property expressed by the matrix clause [Wiltschko 95]. The discourse referent is in some way related to a previously established set which the referent is a member of. This resembles the presuppositions restrictive relative clauses establish. As for a hanging topic, it also indicates a topic shift. It introduces a new topic of the discourse from a set of discourse referents that have already been established in the discourse. The common property of shifting the discourse topic implies that hanging topics and left dislocations are not mutually exclusive. A distinction on pragmatic grounds is complicated by the fact that the various set phrases usable for the hanging topic can have different discourse functions and that left dislocations can be interpreted as special hanging topics. However, the main difference between left dislocations and hanging topics with the set phrase was das X betrifft (“as for the X”) seems to be: left dislocations must satisfy the presupposition condition and they establish a topic shift by means of changing the status of a discourse referent, whereas hanging topics establish a topic shift by means of selecting a discourse referent from a previously established set of referents.3 Despite their overlapping discourse functions, we confine ourselves to the distinctive pragmatic properties of both constructions for their generation. Hence, the second problem that needs to be solved is to correlate the syntactic form with the status of discourse referents with respect to their being active or accessible, as well as with other discourse information and factual information pertaining to the presupposition conditions. How can we incorporate 3See the extensive descriptive analyses in [Altmann 81]. this informal characterization of topic, topic acceptability, and syntactic constructions into a unified and formally precise mechanism for a natural language generation system? We propose an abductive setting in the spirit of [Hobbs et al. 93] as a framework for integrating the diverse knowledge sources involved in the generation and interpretation of sentential information structure. The basic idea is to view generating a single proposition as finding the best proof for why a sentence and its information structure is congruent with the listener model. In the process of finding this proof, the sentence is generated by incrementally instantiating unbound variables. Our basic scenario is the generation of spatial descriptions. The mechanism for content planning is not the subject of this paper (cf. [Jansche et al. 96, Meyer-Klabunde 96, Porzel et al. in press]). For spatial descriptions, content planning comprises for each proposition the selection of a reference object from the set of objects, the selection of a primary object, the selection of a point of view, and the computation of a spatial relation between both objects depending on the chosen point of view. For present purposes we assume that the propositional content of a sentence has already been established. What remains to be done is to construct a pragmatically appropriate sentence that conveys the new and informative part of this propositional information to the listener. It is for this syntactic/pragmatic realization process that we use the abductive framework. Ultimately, we aim to incorporate the abductive reasoning mechanism directly into the content planner so as to achieve a uniform framework. 3 Generation by Abduction Abductive reasoning is reasoning about the best explanation for a given observation. To make precise what counts as a good explanation, one introduces a preference criterion by which alternative explanations can be compared. A preferred explanation for an observation might be the least specific one, the most specific one, the one with the lowest proof costs, etc. Abductive explanation is classically characterized as follows (cf. [Mayer et al. 96]): a knowledge base K, the usual consequence relation , and an observation E to be explained, such that K 2 E, are given. A statement H is taken as the best explanation of E in K iff: