Tense Trees as the "Fine Structure" of Discourse

We present a new compositional tense-aspect deindexing mechanism that makes use of tense trees as components of discourse contexts. The mechanism allows reference episodes to be correctly identified even for embedded clauses and for discourse that involves shifts in temporal perspective, and permits deindexed logical forms to be automatically computed with a small number of deindexing rules. 1 I n t r o d u c t i o n Work on discourse structure, e.g., [Reichman, 1985; Grosz and Sidner, 1986; Allen, 1987], has so far taken a rather coarse, high-level view of discourse, mostly treating sentences or sentence-like entities ("utterance units, . . . . contributions," etc.) as the lowest-level discourse elements. To the extent that sentences are analyzed at all, they are simply viewed as carriers of certain features relevant to supra-sentential discourse structure: cue words, tense, time adverbials, aspectual class, intonational cues, and others. These features are presumed to be extractable in some straightforward fashion and provide the inputs to a higher-level discourse segment analyzer. However, sentences (or their logical forms) are not in general "flat," with a single level of structure and features, but may contain multiple levels of clausal and adverbial embedding. This substructure can give rise to arbitrarily complex relations among the contributions made by the parts, such as temporal and discourse relations among subordinate clausal constituents and events or states of affairs they evoke. It is therefore essential, in a comprehensive analysis of discourse structure, that these intra-sentential relations be systematically brought to light and integrated with larger-scale discourse structures. Our particular interest is in tense, aspect and other indicators of temporal structure. We are developing a uniform, compositional approach to interpretation in which a parse tree leads directly (in rule-to-rule fashion) to a preliminary, indezical logical form, and this LF is deindezed by processing it in the current context (a well-defined structure). Deindexing simultaneously transforms the LF and the context: context-dependent constituents of the LF, such as operators past, pres and perf and adverbs like today or earlier, are replaced by explicit relations among quantified episodes; (anaphora are also deindexed, but this is not discussed here); and new structural components and episode tokens (and other information) are added to the context. This dual transformation is accomplished by simple recursive equivalences and equalities. The relevant context structures are called tense trees; these are what we propose as the "fine structure" of discourse, or at least as a key component of that fine structure. In this paper, we first review Reichenbach's influential work on tense and aspect. Then we describe temporal deindexing using tense trees, and extensions of the mechanism to handle discourse involving shifts in temporal perspective. 2 Farewel l to R e i c h e n b a c h Researchers concerned with higher-level discourse structure, e.g., Webber [1987; 1988], Passonneau [1988] and Song and Cohen [1991], have almost invariably relied on some Reichenbach [1947]-1ike conception of tense. The syntactic part of this conception is that there are nine tenses in English, namely simple past, present and future tense, past, present and future perfect tense, and posterior past, present and future tense 1 (plus progressive variants). The semantic part of the conception is that each tense specifies temporal relations among exactly three times particular to a tensed clause, namely the event time (E), the reference time (R) and the speech time (S). On this conception, information in discourse is a matter of "extracting" one of the nine Reichenbachian tenses from each sentence, asserting the 1Exarnples of expressions in posterior tense are would, was going to (posterior past), is going to (posterior present), and will be going to (posterior future).