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<?xml version="1.0" standalone="yes"?> <Paper uid="J88-2006"> <Title>TENSE AS DISCOURSE ANAPHOR</Title> <Section position="4" start_page="0" end_page="0" type="metho"> <SectionTitle> 3 TENSE AS ANAPHOR </SectionTitle> <Paragraph position="0"> Tense may not seem prima facie anaphoric: an isolated sentence like John went to bed or I met a man who looked like a basset hound appears to make sense in a way that a stand-alone He went to bed or The man went to bed does not. On the other hand, if some time or event is established by the context (i.e., either by an event or situation described in the previous discourse or by a temporal adverbial in the current sentence--cf.</Paragraph> <Paragraph position="1"> Passonneau, and Moens and Steedman, this volume), tense will invariably be interpreted with respect to it, as in: 7. a. After he finished his chores, John went to bed.</Paragraph> <Paragraph position="2"> b. John partied until 3 am. He came home and went to bed.</Paragraph> <Paragraph position="3"> In each case, the interpretation of John's going to bed is linked to an explicitly mentioned time or event. This is what underlies all discussion of the anaphoric quality of tense.</Paragraph> <Section position="1" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 3.1 BACKGROUND </SectionTitle> <Paragraph position="0"> The assumption that tense is anaphoric (i.e., that its interpretation is linked to some time or event derived from context) goes back many years, although it is not a universally held belief (cf. Comrie 1985). Leech seems to express this view in his Meaning and the English Verb:</Paragraph> </Section> </Section> <Section position="5" start_page="0" end_page="0" type="metho"> <SectionTitle> 63 INDEFINITE TIME Whereas the Present Per- </SectionTitle> <Paragraph position="0"> fect, in its indefinite past sense, does not name a specific point of time, a definite POINT OF ORI-ENTATION in the past is normally required for the appropriate use of the Simple Past Tense. The point of orientation may be specified in one of three ways: (a) by an adverbial express of timewhen; (b) by a preceding use of a Past or Perfect Tense; and (c) by implicit definition; i.e., by assumption of a particular time from context.</Paragraph> <Paragraph position="1"> 73 Tile Past Perfect Tense has the meaning of pastin-the-past, or more accurately, 'a time further in the past, seen from the viewpoint of a definite point of time already in the past'. That is, like the Simple Past Tense, the Past Perfect demands an already established past point of reference.</Paragraph> <Paragraph position="2"> (Leech: 47) Leech did not elaborate further on how reference points are used in the interpretation of simple past tense and past perfect tense, or on what has become the main problem in the semantics and pragmatics of tense: reconciling the (usual) forward movement of events in narratives with a belief in the anaphoric (or contextdependent) character of tense.</Paragraph> <Paragraph position="3"> The first explicit reference I have to tense being anaphoric like a definite pronoun is in an article by McCawley (1971:110), who said: However the tense morpheme does not just express the time relationship between the clause it is in and the next higher clause--it also refers to the time of the clause that it is in, and indeed, refers to it in a way that is rather like the way in which personal pronouns refer to what they stand for.</Paragraph> <Paragraph position="4"> McCawley also tried to fit in his view of tense as pronoun with the interpretation of tense in simple narratives. Here he proposed that the event described in one clause serves as the antecedent of the event described in the next, but that it may be related to that event by being either at the same time or &quot;shortly after&quot; it. He did not elaborate on when one relation would be assumed and when the other.</Paragraph> <Paragraph position="5"> Partee (1973) also noted the similarities between tense and definite pronouns. However, she subsequently recognized that taking simple past tense as directly analogous with pronouns was incompatible with the usual forward movement of time in the interpretation in a sequence of sentences denoting events (Partee 1984). Her response was a modification of the claim that tense is anaphoric, saying: I still believe it is reasonable to characterize tense as anaphoric, or more broadly as context-dependent, but I would no longer suggest that this requires them to be viewed as 'referring' to times as pronouns 'refer' to entities, or to treat times as arguments of predicates (256).</Paragraph> <Paragraph position="6"> The particular context-dependent process she proposes for interpreting tensed clauses follows that of Hinrichs 1986, briefly described below.</Paragraph> <Paragraph position="7"> The examples presented above to illustrate the anaphoric quality of tense were all simple past. However, as Leech notes (see above), the past perfect also makes demands on having some reference point already estab64 Computational Linguistics, Volume 14, Number 2, June 1988 Bonnie Lynn Webber Tense as Discourse Anaphor lished in the context. Thus it cannot be in terms of the event described in a tensed clause that tense is anaphoric. Instead, several people (Steedman 1982, Hinrichs 1986, Bauerle 1979) have argued that it is that part of tense called by Reichenbach (1947) the point of reference (here abbreviated RT) that is anaphoric. This can be seen by considering the following example: 8. a. John went to the hospital.</Paragraph> <Paragraph position="8"> b. He had twisted his ankle on a patch of ice.</Paragraph> <Paragraph position="9"> It is not the point of the event (here abbreviated ET) of John's twisting his ankle that is interpreted anaphorically with respect to his going to the hospital. Rather, it is the RT of the second clause: its ET is interpreted as prior to that because the clause is in the past perfect (see above).</Paragraph> <Paragraph position="10"> I will now review briefly Hinrichs's proposal as to how tensed clauses are interpreted in context, in order to contrast it with the current proposal.</Paragraph> <Paragraph position="11"> In Hinrichs 1986, Hinrichs makes the simplifying assumption that in a sequence of simple past sentences, the temporal order of events described cannot contradict the order of the sentences. This allows him to focus on the problem of characterizing those circumstances in which the event described by one sentence follows that described by the previous one (Example 9---Hinrichs's Example 15) and when it overlaps it (Example 10---Hinrichs's Example 21): 9. The elderly gentleman wrote out the check, tore it from the book, and handed it to Costain.</Paragraph> <Paragraph position="12"> 10. Mr. Darby slapped his forehead, then collected himself and opened the door again. The brush man was smiling at him hesitantly.</Paragraph> <Paragraph position="13"> Hinrichs bases his account on the Aktionsart of a tensed clause (i.e., its Vendlerian classification as an accomplishment, achievement, activity, or state--including progressives). Assuming an initial reference point in a discourse, the event described by a tensed clause interpreted as an accomplishment or achievement will be included in that reference point and will also introduce a new reference point ordered after the old one. Events associated with the other Aktionsarten include the current reference point in the event time. This means that given a sequence of two clauses interpreted as accomplishments or achievements, their corresponding events will follow one another (cf. Example 9). On the other hand, given a sequence with at least one tensed clause interpreted as an activity or state (including progressive), their corresponding events will be interpreted as overlapping each other (cf. Example 10).</Paragraph> <Paragraph position="14"> Hinrichs relates his reference point to that of Reichenbach. (Thus, the anaphoric character of tense is based on RT and not on the events directly.) However, Hinrichs's notion and Reichenbach's differ with respect to the time of the event described in the tensed clause. While Reichenbach talks about ET and RT being the same for nonprogressive past-tense clauses, in Hinrichs's account the reference point can fall after the event if a nonprogressive past is interpreted as an accomplishment or an achievement. This is necessary to achieve the forward movement of narrative that Hinrichs assumes is always the case (his simplifying assumption) but it is not the same as Reichenbach's RT.</Paragraph> <Paragraph position="15"> It also leads to problems in cases where this simplifying assumption is just wrong--where in a sequence of simple past tenses, there is what appears to be a &quot;backward&quot; movement of time, as in 11. a. For an encore, John played the &quot;Moonlight Sonata&quot;.</Paragraph> <Paragraph position="16"> b. The opening movement he took rather tentatively, but then...</Paragraph> <Paragraph position="17"> where the second clause should be understood as describing the beginning of the playing event in more detail, not as describing a subsequent event.</Paragraph> <Paragraph position="18"> In the account given below, both forward and backward movement of time fall out of the anaphoric character of tensed clauses, and the dependency of discourse anaphora on discourse structure.11</Paragraph> </Section> <Section position="6" start_page="0" end_page="0" type="metho"> <SectionTitle> 3.2 TENSE AS DISCOURSE ANAPHOR: IN WHAT SENSE &quot;SPECIFY&quot;? </SectionTitle> <Paragraph position="0"> With that background, I will now show how tensed clauses share the two properties I set out in Section 1 (repeated here) and hence are further examples of discourse anaphora: 1. Anaphors specify entities in an evolving model of the discourse that the listener is constructing; 2. the particular entity specified depends on another entity in that part of the evolving Discourse Model that the listener is currently attending to. To do this, I need to explain the sense in which tensed clauses specify and the way in which that specification can depend on another element in the current context. Recall that I presume that a listener's developing discourse model represents both the entities being discussed, along with their properties and relations, and the events and situations being discussed, along with their relationships with another. For the rest of this paper, I want to ignore the former and focus on the latter. This I will call event/situation structure, or E/S structure. It represents the listener's best effort at interpreting the speaker's ordering of those events and situations in time and space. One problem in text understanding, then, is that of establishing where in the evolving E/S structure to integrate the event or situation description in the next clause.</Paragraph> <Paragraph position="1"> In this framework, a tensed clause C b provides two pieces of semantic information: (a) a description of an event or situation, and (b) a particular configuration of ET, RT, and point of speech (abbreviated ST). (Here I may be departing from Reichenbach in treating ET, RT, and ST explicitly as elements of linguistic semantics, quite distinct from entities of type &quot;event&quot; in the Computational Linguistics, Volume 14, Number 2, June 1988 65 Bonnie Lynn Webber Tense as Discourse Anaphor Discourse Model.) Cb then specifies an entity E b in the Discourse Model whose temporal relationship to other events in the model follows (in part) from Cb'S particular configuration of ET, RT, and ST. Both the characteristics of E b (i.e., its ontology) and the configuration of ET, RT, and ST are critical to my account of tense as discourse anaphor.</Paragraph> <Paragraph position="2"> The event ontology I assume follows that of Moens and Steedman (this volume) and of Passonneau (this volume). Both propose that people interpret events as having a tripartite structure (a &quot;nucleus&quot; in Moens and Steedman's terminology) consisting of a preparatory phase (prep), a culmination (cuD, and a consequent phase (conseq)--as in Figure 2. This tripartite structure permits a uniform account to be given of aspectual types in English and of how the interpretation of temporal adverbials interacts with the interpretation of tense and aspect. For example, the coercion of clauses from one interpretation to another is defined in terms of which parts of a nucleus they select and how those parts are described. 12 prep cul conseq (lllllllllllllltllllllllllllll ~ Figure 2. Tripartite ontology of events.</Paragraph> <Paragraph position="3"> The ET/RT/ST configuration is significant in that, like Steedman 1982, Dowty 1986, Hinrichs 1986, and Partee 1984, I take RT as the basis for anaphora. To indicate this, I single it out as an independent argument to anaphoric functions, here labelled/3. In particular, the following schema holds of a clause C b linked anaphorically to an event Ea through its RT: /3(Cb, Ea, RTb) = E b The relationship between E b and Ea then falls out as a consequence of I. the particular ET/RT/ST configuration of Cb; and 2. the particular function/3 involved.</Paragraph> <Paragraph position="4"> One possibility is that/3 links RT b directly to Ea--i.e.: null /30(Cb, Ea, RTb) = E b In this case, the relationship between E b and Ea then depends on the configuration of RT b and ET b. If ET b = RTb, then (minimally) E b is taken to coincide in some way with E a. This is shown in Figure 3a. If ET b < RT b (as in the perfect tenses), E b is taken to precede Ea. This is shown in Figure 3d.</Paragraph> <Paragraph position="5"> Alternatively, /3 may embody part of the tripartite ontology of events mentioned earlier:/3prep links RT b to the preparatory phase of E a (as shown in Figure 3b)-- null while/3conseq links RT b to the consequent phase of E a (as shown in Figure 3c)--i.e.: /3conseq(Cb, E a, RTb) = Eb (There is a third possibility--that RTb links to the culmination of Ea--but it is not clear to me that it could be distinguished from the simpler /3o function given above, which links RT b to Ea itself. Also, while /3prep and /3co,,seq relations for RTb might theoretically be possible for a perfect, it is not clear to me that these cases could be distinguished from the simpler/3o- In the case of perfects therefore, the relation between E b and Ea is correspondingly indirect.'3 The following example illustrates the case where/3 = /3 o and ETb = RTb.</Paragraph> <Paragraph position="6"> 12. a. John played the piano.</Paragraph> <Paragraph position="7"> b. Mary played the kazoo.</Paragraph> <Paragraph position="8"> Sentence 12a. evokes a new event entity Ea describable as the event of John playing the piano. Since the tense of (12b) is simple past, ET b = RTb. Given /3o(Cb,Ea,</Paragraph> <Paragraph position="10"> Ea. (Whether this is further interpreted as two simultaneous events or a single event of their playing a duet depends on context and, perhaps, world knowledge as well.) This is illustrated in Figure 4. Example 8 (repeated here) illustrates the case/30 where ET b < RT b.</Paragraph> <Paragraph position="11"> i Now 8. a. John went to the hospital.</Paragraph> <Paragraph position="12"> b. He had twisted his ankle on a patch of ice. Clause 8a. evokes an entity E a describable as John's going to the hospital. Since 8b is past perfect, ET b < RT b. Thus if /3o(Cb,Ea,RTb) = Eb, the event E b de- null scribed by 8b is taken to be prior to E a. As Moens & Steedman (this volume) point out, the consequences of an event described with a perfect tense are still assumed to hold. Hence the overlap shown in Figure 5:</Paragraph> <Paragraph position="14"> The next example illustrates /3conseq: 13. a. John went into the florist shop.</Paragraph> <Paragraph position="15"> b. He picked out three red roses, two white ones and one pale pink.</Paragraph> <Paragraph position="16"> Clause 13a evokes an entity E a describable as John's going into a flower shop. Since Clause 13b is simple past, ET b = RT b. Thus given/3conseq(Cb,Ea,RT b) = E b, event E b is taken as being part of the consequent phase of Ea. That is, John's picking out the roses is taken as happening after his going into the florist shop. This is shown in Figure 6. The next example illustrates the case of/3prep: 14. a. John bought Mary some flowers.</Paragraph> <Paragraph position="17"> b. He picked out three red roses, two white ones and one pale pink.</Paragraph> <Paragraph position="18"> Since 14b is simple past, ET b = RT b. Thus given</Paragraph> <Paragraph position="20"> out some roses--is taken as being part of the preparatory phase of the event E a, which when completed, can</Paragraph> <Paragraph position="22"> be described as having bought some flowers. This is shown in Figure 7.</Paragraph> <Paragraph position="24"> To summarize, I have claimed that: 1. the notion of specification makes sense with respect to tensed clauses; 2. one can describe the anaphoric relation in terms of the RT of a tensed clause Cb, its ET/RT configuration, and an existing event or situation entity E~----that is, /3(Cb,Ea,RT b) = Eb; and 3. there are (at least) three /3 functions---one, /3o, linking RT b to E a itself, the other two (/3prep and/3conseq) embodying parts of a tripartite ontology of events. In the next section, I will discuss constraints on the second argument to /3(Cb,Ea,RTb)--that is, constraints on which entities in the evolving E/S structure the specification of a tensed clause can depend on.</Paragraph> <Section position="1" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 3.3 TEMPORAL FOCUS </SectionTitle> <Paragraph position="0"> Recall from Section 2.2 that Sidner introduced the notion of a dynamically changing discourse focus (DF) to capture the intuition that at any point in the discourse, there is one discourse entity that is the prime focus of attention and that is the most likely (although not the only possible) specificand of a definite pronoun. In parallel, I propose a dynamically changing temporal focus (TF), to capture a similar intuition that at any point in the discourse, there is one entity in E/S structure that is most attended to and hence most likely to stand in an anaphoric relation with the RT of the next clause. That is, fl(Cb,TF,RT b) = E b. If Cb is interpreted as part of the current discourse segment, after its interpretation there are three possibilities: I. With/30, the TF will stay where it is, independent of whether ET = RT or ET < RT.</Paragraph> <Paragraph position="1"> 2. With/3conseq, RTb'S link to the consequent phase of the TF locates event E b there, shifting the TF forward (to Eb). This is the &quot;natural&quot; forward progression of narrative.</Paragraph> <Paragraph position="2"> 3. With/3prep, RTb'S link to the preparatory phase of the TF locates E b there, shifting the TF backward to (Eb). This is used to elaborate an event or situation in more detail.</Paragraph> <Paragraph position="3"> These relationships, which I will call maintenance and local movement of the TF, correspond to Sidner's DF moving gradually among the discourse entities in a discourse segment. (They cover the same phenomena as the micromoves that Nakhimovsky describes in his paper (this volume).) More radical movement of TF correspond to changes in discourse structure. (These Computational Linguistics, Volume 14, Number 2, June 1988 67 Bonnie Lynn Webber Tense as Discourse Anaphor cover similar phenomena to the macromoves described in Nakhimovsky, also this volume.) In cases involving movements into and out of an embedded discourse segment, either 1. the TF will shift to a different entity in E/S structure---either an existing entity or one created in recognition of an embedded narrative; or 2. it will return to the entity previously labeled TF, after completing an embedded narrative. Such movements are described in Section 3.3.2. Other movements, signaled by temporal adverbials and when clauses, are not discussed in this paper. 14</Paragraph> </Section> </Section> <Section position="7" start_page="0" end_page="68" type="metho"> <SectionTitle> 3.3.1 TEMPORAL FOCUS: MAINTENANCE AND LOCAL MOVEMENT </SectionTitle> <Paragraph position="0"> The following pair of examples illustrate maintenance and local movement of TF within a discourse segment and its link with E/S structure construction. The first I discussed in the previous section to illustrate /3conseq&quot; The second is a variation on that example: 13. a. John went into the florist shop.</Paragraph> <Paragraph position="1"> b. He picked out three red roses, two white ones and one pale pink.</Paragraph> <Paragraph position="2"> 15. a. John went into the florist shop.</Paragraph> <Paragraph position="3"> b. He had promised Mary some flowers.</Paragraph> <Paragraph position="4"> c. He picked out three red roses, two white ones, and one pale pink.</Paragraph> <Paragraph position="5"> First consider Example 13. The first clause (13a) evokes an event entity E a describable as John's going into the florist shop. Since its tense is simple past, E a is interpreted as prior to ST. Since it begins the discourse, its status is special vis-~t-vis both definite NPs and tensed clauses. That is, since no previous TF will have been established yet, the listener takes that entity E a to serve as TF. 15 This is shown in Figure 8: Partee, and Dowty were out to achieve. Here it falls out simply :from the discourse notion of a TF and from the particular anaphoric function/3C/o,scq.16 Now consider Example 15 (repeated here) whose first clause is the same as Example 13a and hence would be processed in the same way.</Paragraph> <Paragraph position="6"> The tense of the next clause (15b) is past perfect. As I noted above, the only anaphoric function on RT15b and an event entity that makes sense for perfect tenses is</Paragraph> <Paragraph position="8"> Given that perfect tenses imply ET < RT, the event E b specified by (15b) will be interpreted as being prior to E a. Moreover, since (15b) is past perfect, the consequent phase of E b is assumed to still hold with respect to If Clause 13b is interpreted as being part of the same discourse segment as (13a) it must be the case that fl(C13b,TF,RT136). Assume the listener takes /3 to be /3cons~,q on the basis of world knowledge--that is, /3conscq(Clab,TF,RT13b). Since the tense of (13b) is simple past, its RT and ET coincide. Thus (13b) specifies a new entity Eb, located within the consequent phase of the TF--that is, Ea--and hence after it. I assume that, following the computation of the anaphoric function, TF becomes associated with the event entity located at RT b. In this case, it is E b, and TF thereby moves forward (cf. Figure 9). As noted, this is the gradual forward movement of simple narratives that Hinrichs, Now Clause 15c is the same as (13b), and TF is the same as it was at the point of interpreting (13b). Thus not surprisingly, 15c produces the same change in E/S Computational Linguistics, Volume 14, Number 2, June 1988 Bonnie Lynn Webber Tense as Discourse Anaphor structure and in the TF as (13b), resulting in the diagram shown in Figure 11.</Paragraph> </Section> <Section position="8" start_page="68" end_page="68" type="metho"> <SectionTitle> 3.3.2 TEMPORAL FOCUS: DISCOURSE-STRUCTURE MOVEMENTS </SectionTitle> <Paragraph position="0"> To illustrate the effect of discourse structure on TF, consider the following variation on Example 15, which had the same structure vis-~t-vis sequence of tenses.</Paragraph> <Paragraph position="1"> 16. a. John went into the florist shop.</Paragraph> <Paragraph position="2"> b. He had promised Mary some flowers.</Paragraph> <Paragraph position="3"> c. She said she wouldn't forgive him if he forgot.</Paragraph> <Paragraph position="4"> The first two clauses (a) and (b) are the same as in Example 15 and lead to the same configuration of event entities in E/S structure (as shown in Figure 10). But the most plausible interpretation of (16c) is where the &quot;saying&quot; event is interpreted anaphorically with respect to the &quot;promising&quot; event--that is, where (16b-c) are taken together as (the start of) an embedded discourse, describing an event prior to John's going to the florist's. To handle this, I assume, following Grosz and Sidner 1986, that when the listener recognizes an embedded discourse segment, s/he stores the current TF for possible resumption later. '7 However, I also assume the listener recognizes the embedding not when s/he first encounters a perfect-tensed clause Cb, since it needn't signal an embedded discourse, but later, when an immediately following simple past tense clause Cc is most sensibly interpreted with respect to the event entity E b that Cb evoked. ~8 At this point, the listener moves TF from its current position to E b, caching the previous value for possible resumption later. Following this gross movement, /3(Cc,TF,RT~) will be computed.</Paragraph> <Paragraph position="5"> If/3 is then interpreted as /3conseq or/3prep, there will be a second movement of TF.'9 Coming back to Example 16, if Clause 16c is taken as being part of a single discourse segment with (16a-b), she saying something would have to be interpreted with respect to the current TF (Ea)---John's going to the florist. This is implausible under all possible interpretations of/3.2o However, under the assumption that E b is part of an embedded narrative, the listener can a posteriori shift TF to E b and consider the anaphoric relation /3(C16c, TF, RT16c) = El6 c with Eb as TF. At this point, the listener can plausibly take/3 to be/3co,seq based on world knowledge. Since (16c) is simple past, ETc = RT~, the &quot;saying&quot; event E~ is viewed as part of the consequent phase (and hence following) the &quot;promising&quot; event E b. As in the first case, TF moves to the event located at RT~--i.e., to Ec.</Paragraph> <Paragraph position="6"> This is shown roughly in Figure 12. Notice that this involved two movements of TF---once in response to a perceived embedded segment and a second time, in response to interpreting/3 as/3C/o,seq-Now consider the following extension to (16): 17. a. John went into the florist shop.</Paragraph> <Paragraph position="7"> b. He had promised Mary some flowers.</Paragraph> <Paragraph position="8"> c. She said she wouldn't forgive him if he forgot.</Paragraph> <Paragraph position="9"> d. So he picked out three red roses, two white ones, and one pale pink.</Paragraph> <Paragraph position="10"> As before, Clauses 17b-c form an embedded narrative, but here the main narrative of John's visit to the florist shop, started at (17a), is continued at (17d). To handle this, I again assume that TF behaves much like Sidner's DF in response to the listener's recognition of the end of an embedded narrative: that is, the cached TF is resumed and processing continues. 2~ Figure 12. E/S structure after processing Clause 16c.</Paragraph> <Paragraph position="11"> Under this assumption, Clauses 17a-c are interpreted as in the previous example (cf. Figure 12). Recognizing Clause 17d as resuming the embedding segment, 2z the previously cached TF (Ea---the going into the florist shop event) is resumed. Again assume that the listener takes the anaphoric function to be flconseq(Cd,TF,RTd) = E o on the basis of world knowledge. Since Clause 17d is simple past (ET = RT), the picking out roses event E d is viewed as part of the consequent phase and hence following the going into the florist shop event. This is shown roughly in Figure 13: Now getting the listener to interpret a text as an embedded narrative requires providing him/her with another event or situation that TF can move to. One way in English is via a perfect-tensed clause, which Computational Linguistics, Volume 14, Number 2, June 1988 69 Bonnie Lynn Webber Tense as Discourse Anaphor explicitly evokes another event, temporally earlier than the one currently in focus. Another way is by lexical indications of an embedded narrative, such as verbs of telling and NPs that themselves denote events or situations (e.g., ones headed by de-verbal nouns).</Paragraph> <Paragraph position="12"> This is illustrated in Example 18. Even though all its clauses are simple past (ET = RT), Clauses 18c-d are most plausibly interpreted as indirect speech describing an event that has occurred prior to the &quot;telling&quot; event. I assume that in response to recognizing this kind of embedded narrative, the listener creates a new node of E/S structure and shifts TF there, caching the previous value of TF for possible resumption later. The temporal location of this new node vis-a-vis the previous TF will depend on information in the tensed clause and on the listener's world knowledge.</Paragraph> <Paragraph position="13"> 18. a. I was at Mary's house yesterday.</Paragraph> <Paragraph position="14"> b. We talked about her sister Jane.</Paragraph> <Paragraph position="15"> c. She spent five weeks in Alaska with two friends.</Paragraph> <Paragraph position="16"> d. Together, they climbed Mt. McKinley.</Paragraph> <Paragraph position="17"> e. Mary asked whether I would want to go to Alaska some time.</Paragraph> <Paragraph position="18"> Notice that, as with embedded narratives cued by the use of a perfect tense, caching the previous TF for resumption later enables the correct interpretation of Clause 18e, which is most plausibly interpreted as following the telling about her sister event.</Paragraph> <Paragraph position="19"> An NP denoting an event or situation (such as one headed by a noun like trip or by a de-verbal noun like installation) can also signal the upcoming possibility of an embedded narrative that will elaborate that event or situation (past, upcoming, or hypothetical) in more detail, as in Example 19. In this case, the original NP and the subsequent clause(s) will be taken as co-specifying the same thing. The question here is how and when TF moves.</Paragraph> <Paragraph position="20"> 19. a. I was talking with Mary yesterday.</Paragraph> <Paragraph position="21"> b. She told me about her trip to Alaska.</Paragraph> <Paragraph position="22"> c. She spent five weeks above the Arctic Circle with two friends.</Paragraph> <Paragraph position="23"> d. The three of them climbed Mt. McKinley.</Paragraph> <Paragraph position="24"> After interpreting Clause 19b, the TF is at the &quot;telling&quot; event. I claim that the NP her trip to Alaska, while evoking a discourse entity, does not affect the TF. If Clause 19c is interpreted as the start of an embedded narrative (as it is here), TF moves to the event entity E c it evokes (caching the previous value Eb). At this point, using additional reasoning, the listener may recognize an anaphoric relation between Clause 19c and the discourse entity evoked by her trip to Alaska. Support for this, rather than assuming that an event-denoting NP sets up a potential focus, just as I claim a perfect-tensed clause does, comes from the reasoning required to understand the following parallel example, where I would claim TF does not move.</Paragraph> <Paragraph position="25"> 20. a. I was talking with Mary yesterday.</Paragraph> <Paragraph position="26"> b. She told me about her trip to Alaska.</Paragraph> <Paragraph position="27"> c:. She had spent five weeks above the Arctic Circle with two friends.</Paragraph> <Paragraph position="28"> d. The three of them had climbed Mt. McKinley.</Paragraph> <Paragraph position="29"> e. She said that next year they would go for Aconcagua.</Paragraph> <Paragraph position="30"> The event described in Clause 20c is the same as that described in Clause 19c, and should be interpreted anaphorically with respect to the entity her trip to Alaska in the same way. If this is the case, however, then the anaphoric link does not follow from the movement of TF.</Paragraph> </Section> <Section position="9" start_page="68" end_page="68" type="metho"> <SectionTitle> 3.3.3 TEMPORAL FOCUS: MISCELLANY </SectionTitle> <Paragraph position="0"> Example 20 above illustrates one case of an anaphoric function on an NP and a tensed clause, specifically fl(Cb,Ea,RTb) where the entity Ea has been evoked by an NP rather than a clause. Another possibility is that a(NPb,Ea) = Eb, where NP b is definite by virtue of an entity evoked by a clause rather than an NP that is, Eb, is associated with either the preparatory/culmination/consequent structure of Ea, as in 21. a. Mary climbed Mt. McKinley.</Paragraph> <Paragraph position="1"> b. The preparations took her longer than the ascent.</Paragraph> <Paragraph position="2"> or its associated role structure, as in 22. a. John bought a television.</Paragraph> <Paragraph position="3"> b. Although he had intended to buy a 13&quot; b/w set, the salesman convinced him to buy a 25&quot; color, back-projection job.</Paragraph> <Paragraph position="4"> where the salesman fills a particular role in the buying event.</Paragraph> <Paragraph position="5"> Next, notice that ambiguities arise when there is more than one way to plausibly segment the discourse, as in the following example: 23. a. I told Frank about my meeting with Ira.</Paragraph> <Paragraph position="6"> b. We talked about ordering a Butterfly.</Paragraph> <Paragraph position="7"> Here it is plausible to take Clause 23b as the beginning of an embedded narrative, whereby the &quot;talking about&quot; event is interpreted against a new node of E/S structure, situated prior to the &quot;telling Frank&quot; event. (In this case, we is Ira and me.) It is also plausible to take (23b) as continuing the current narrative, whereby the &quot;talking about&quot; event is interpreted with respect to the &quot;telling Frank&quot; event. (In contrast here, we is Frank and me.) Finally, consider things from the point of view of generation. If some event E b is part of the preparatory phase of some event Ea, and a description of Ea has just been generated using the simple past tense, then E b could be described using either the simple past, as in Example 24 or past perfect, as in Example 25.</Paragraph> <Paragraph position="8"> 70 Computational Linguistics, Volume 14, Number 2, June 1988 Bonnie Lynn Webber Tense as Discourse Anaphor 24. a. John went to the hospital.</Paragraph> <Paragraph position="9"> b. He took a taxi, because his car was in the shop 25. a. John went to the hospital.</Paragraph> <Paragraph position="10"> b. He had taken a taxi, because his car was in the shop.</Paragraph> <Paragraph position="11"> In the case of Example 24, the listener/reader recognizes that Eb is part of the preparatory phase of E a and that Eb therefore precedes E a. In the case of Example 25, the listener would first recognize that E b precedes Ea because of the past perfect, but then recognize Eb as part of the preparatory phase of E a.</Paragraph> <Paragraph position="12"> On the other hand, if E b simply precedes E a, but a description of E a has been generated first, then Eb must be described with a past perfect (Example 26): simple past would not be sufficient (Example 27).</Paragraph> <Paragraph position="13"> 26. a. John went to the hospital.</Paragraph> <Paragraph position="14"> b. He had broken his ankle, walking on a patch of ice.</Paragraph> <Paragraph position="15"> 27. a. John went to the hospital.</Paragraph> <Paragraph position="16"> b. *He broke his ankle, walking on a patch of ice.</Paragraph> </Section> class="xml-element"></Paper>