Discourse Relations: A Structural and Presuppositional Account Using Lexicalised TAG

Discourse Relations: A Structural and Presuppositional Account Using Lexicalised TAG* Bonnie Webber Univ of Edinburgh bonnie@dai.ed.ac.uk Alistair Knott Univ of Otago alik@cs.otago.ac.nz Matthew Stone Rutgers Univ mdstone@cs.rutgers.edu Aravind Joshi Univ of Pennsylvania joshi @cis.upenn.edu Abstract We show that discourse structure need not bear the full burden of conveying discourse relations by showing that many of them can be explained non- structurally in terms of the grounding of anaphoric presuppositions (Van der Sandt, 1992). This simpli- fies discourse structure, while still allowing the real- isation of a full range of discourse relations. This is achieved using the same semantic machinery used in deriving clause-level semantics. 1 Introduction Research on discourse structure has, by and large, attempted to associate all meaningful relations between propositions with structural connections between discourse clauses (syntactic clauses or structures composed of them). Recognising that this could mean multiple structural connections between clauses, Rhetorical Structure Theory (Mann and Thompson, 1988) simply stipulates that only a single relation may hold. Moore and Pollack (1992) argue that both informational (semantic) and inten- tional relations can hold between clauses simultan- eously and independently. This suggests that factor- ing the two kinds of relations might lead to a pair of structures, each still with no more than a single structural connection between any two clauses. But examples of multiple semantic relations are easy to find (Webber et al., 1999). Having struc- ture account for all of them leads to the complex- ities shown in Figure 1, including the crossing de- pendencies shown in Fig. l c. These structures are no longer trees, making it difficult to define a com- positional semantics. This problem would not arise if one recognised additional, non-structural means of conveying se- mantic relations between propositions and modal * Our thanks to Mark Steedman, Katja Markert, Gann Bierner and three ACL'99 reviewers for all their useful comments. operators on propositions. This is what we do here: Structurally, we assume a "bare bones" dis- course structure built up from more complex elements (LTAG trees) than those used in many other approaches. These structures and the op- erations used in assembling them are the basis for compositional semantics. Stimulated by structural operations, inference based on world knowledge, usage conventions, etc., can then make defeasible contributions to discourse interpretation that elaborate the non- defeasible propositions contributed by com- positional semantics. Non-structurally, we take additional semantic relations and modal operators to be conveyed through anaphoric presuppositions (Van der Sandt, 1992) licensed by information that speaker and hearer are taken to share. A main source of shared knowledge is the interpreta- tion of the on-going discourse. Because the entity that licences (or "discharges") a given presupposition usually has a source within the discourse, the presupposition seems to link the clause containing the presupposition-bearing (p-bearing) element to that source. However, as with pronominal and definite NP anaphora, while attentional constraints on their interpret- ation may be influenced by structure, the links themselves are not structural. The idea of combining compositional semantics with defeasible inference is …

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