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<Paper uid="J88-2006">
  <Title>TENSE AS DISCOURSE ANAPHOR</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
2 BACKGROUND
2.1 DISCOURSE MODELS AND SPECIFICATION
</SectionTitle>
    <Paragraph position="0"> The notion specify that I am using in my definition of discourse anaphora is based on the notion of a Discourse Model, earlier described in Webber (1983). My basic premise is that in processing a narrative text, a listener is developing a model of at least two things: 1. the entities under discussion, along with their properties and relationships to one another, and 2. the events and situations under discussion, along with their relationships to one another (e.g., consequential relations, simple ordering relations, elaboration relations, etc.).</Paragraph>
    <Paragraph position="1"> The representation as a whole I call the listener's Discourse Model. 2 In this section, I will focus on NPs. (In Section 3, I will turn attention to tensed clauses.) NPs may evoke entities into the listener's Discourse Model corresponding to individuals (Example 1), sets (Example 2), abstract individuals (Example 3), classes (Example 4), etc. 3  1. a. Wendy gave Eliot a T-shirt for Christmas.</Paragraph>
    <Paragraph position="2"> b. Unfortunately, (it, the T-shirt) had the logo &amp;quot;You ate it, Ralph&amp;quot;.</Paragraph>
    <Paragraph position="3"> 2. a. Wendy gave each boy a T-shirt.</Paragraph>
    <Paragraph position="4"> b. (They, The T-shirts) each had a different logo on the front.</Paragraph>
    <Paragraph position="5"> 3. a. The vice president must be over 35 years old. b. He or she must also be able to count.</Paragraph>
    <Paragraph position="6"> 4. a. The dachshund down the block bit me  yesterday.</Paragraph>
    <Paragraph position="7"> b. They're really vicious beasts.</Paragraph>
    <Paragraph position="8"> An NP which evokes a discourse entity also specifies it. 4 One way an NP would be considered anaphoric by the above definition would be if it specified an entity E a in the model that had already been evoked by some other NP. (In that case, one would say that the two NPs co-specified the same entity.) This basic arrangement is illustrated in Examples 1-3 above and is shown in Figure la. 5 Formally, one could say that there is an anaphoric function a, whose value, given the anaphoric noun phrase NP b and the discourse entity E a, is Ea-that is, a(NPb,Ea) = E a. This can also be read as NP b specifies E~ by virtue of Ea. Definite pronouns are most often anaphoric in just this way.</Paragraph>
    <Paragraph position="9"> The other way an NP would be considered a discourse anaphor would be if it used some existing discourse entity E a to evoke and specify a new discourse entity Eb, as in  b. I signaled to the driver to stop.</Paragraph>
    <Paragraph position="10"> where NPb--the driver--makes use of the entity associated with the bus mentioned in 5a to specify a new entity--the driver of that bus.</Paragraph>
    <Paragraph position="11"> Here the anaphoric function is of the form a(NPb,Ea) = E b. In cooperative discourse, there have to be constraints on the value of a(NPb,Ea), since only NP b is given explicitly. In short, a cooperative speaker must be able to assume that the listener is able to both infer a possible a and single out Ea in his/her evolving Discourse Model. 6 (This is illustrated in Figure lb.) I will consider each of these two types of constraints in turn. Speakers assume listeners will have no problem with a when a(NPb,E~) = E a. Inferring a in other cases follows in large part from the ontology of the entities specified by NPs--i.e., the ontology of our concepts of individuals, sets, mass terms, generics, etc. We view these as having parts (e.g., car: the engine, the wheels), having functional relations (e.g., car: the driver), having roles (e.g., wedding: the bride), etc. These needn't be necessary parts, relations, roles, etc. Our ontology includes possible parts, relations, etc., and these too make it possible for the listener to infer an a such that a(NPb,Ea) = E b (e.g., room: the chandelier; car: the chauffeur; wedding: the flower girl). Such inferences are discussed at length in the literature, including Clark and Marshall 1981, and Hobbs 1987. 7 Before closing this section, there are two more things to say about NPs. First, the above definition of discourse anaphor does not apply to all definite NPs: a definite NP can be used to refer to something unique in the speaker and listener's shared spatio-temporal context (e.g., the telephone--i.e., the one that they both hear ringing) or their shared culture (e.g., the government), to the unique representative of a class (e.g., the duck-billed platypus), to an entire class or set (e.g., the stars), or to a functionally defined entity (e.g., the 62 Compulational Linguistics, Volume 14, Number 2, June 1988 Bonnie Lynn Webber Tense as Discourse Anaphor largest tomato in Scotland). None of these would be considered discourse anaphoric by the above definition. Secondly, though the definition implies that one must consider some indefinite NPs to be discourse anaphors, since they are essentially parasitic on a corresponding anaphoric definite NP, as in the following example: 6. a. The driver stopped the bus when a passenger began to sing, &amp;quot;Aida&amp;quot;.</Paragraph>
    <Paragraph position="12"> b. The driver stopped the bus when the passengers began to sing &amp;quot;Aida&amp;quot;.</Paragraph>
    <Paragraph position="13"> The indefinite NP a passenger in (6a) can be paraphrased as some one of the passengers, and thus is parasitic on the anaphoric definite NP the passengers mentioned explicitly in (6b). This does not imply that all indefinite NPs are discourse anaphors. In Mary met a boy with green hair or Fred built an oak desk, the indefinite NPs do not need to be interpreted with respect to another discourse entity and some inferrable relationship with that entity, in order to characterize the discourse entity they specify.</Paragraph>
    <Paragraph position="14"> In the next section, I will discuss the second kind of constraint on the function a(NPb,E a) necessary for cooperative use of an anaphor--constraints on identifiable Eas. These involve notions of discourse structure and discourse focus. Before I close, though, I want to point to where I'm going vis-a-vis the anaphoric character of tense and tensed clauses. In contrast with previous accounts of tense as pronoun or tense as loosely context-dependent, I am going to claim that, like an anaphoric definite NP, * a tensed clause C b may either specify an existing event or situation Eb in the listener's Discourse Model, or it may both evoke and specify a new entity.</Paragraph>
    <Paragraph position="15"> * As with anaphoric NPs, there are constraints on possible anaphoric functions and on the Eas that can participate in them at any one time.</Paragraph>
    <Paragraph position="16"> * These functions are sensitive to that part of a tensed clause C b called by Reichenbach (1947) point of reference (here abbreviated RT), as well as its rela- null tionship to Reichenbach's point of the event (ET).</Paragraph>
    <Paragraph position="17"> * These functions can be defined in part in terms of an independently justifiable ontology of events proposed (independently) by Moens and Steedman (this volume) and Passonneau (this volume).</Paragraph>
    <Paragraph position="18"> * The constraints on E a are tied in with a temporal  analogue of discourse focus that I have called temporal focus (TF), and through TF, with discourse structure as well.</Paragraph>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
2.2 DISCOURSE FOCUS AND DISCOURSE STRUCTURE
</SectionTitle>
      <Paragraph position="0"> The ideas presented in this section have been formulated and developed by Barbara Grosz and Candy Sidner, originally independently and later in joint research. It is not a summary of their work: 8 it is limited to those of their ideas that are necessary to the concept of anaphor that I am advancing here and the concept of tense as anaphor, in particular.</Paragraph>
      <Paragraph position="1"> Sidner's thesis (1979, 1983) presents an account of understanding definite pronouns and anaphoric definite NPs that reflects the ease with which people identify the intended specificand of definite pronouns (except in highly ambiguous cases), as well as the intended specificand of anaphoric definite NPs.</Paragraph>
      <Paragraph position="2"> With respect to noun phrases (but not clauses), Sidner makes the same assumption about evoking, specifying, and co-specifying in a Discourse Model that I have made here. To understand anaphoric expressions, Sidner postulates three mechanisms:  1. a current discourse focus (DF) 2. an ordered list of potential foci (PFL) for the next utterance 3. a stack for saving the current DF and resuming it later.</Paragraph>
      <Paragraph position="3">  The DF corresponds to that entity the listener is most attending to. Pronouns can most easily specify the current DF, slightly less easily a member of the PFL, and with slightly more difficulty, a stacked focus. Specifying an entity pronominally can shift the listener's attention to it, thereby promoting it to be the next DF. Anything else specified in the clause ends up on the PFL, ordered by its original syntactic position. (Sidner introduced a separate &amp;quot;agent focus&amp;quot; to allow two entities to be specified pronominally in the same clause, but it was not a critical feature of her approach.) As for anaphoric definite NPs, they can specify anything previously introduced (whether on the PFL, a stacked focus, or anything else) or anything related in a mutually inferrable way with the current DF or a member of the PFL. In terms of the constraints I mentioned above, it is only those discourse entities that are either the DF or on the PFL that can serve as Ea for an anaphoric definite NP. 9 In Sidner (1983) DFs always are stacked for possible resumption later. In Grosz and Sidner (1986) it is an entire focus space (FS) (Grosz 1977) that gets stacked (i.e., the collection of entities L is attending to by virtue of the current discourse segment (DS)) but only when the 9purpose of the current DS is taken to dominate that of the one upcoming. Dominance relations are also specified further according to the type of discourse. In Grosz and Sidner, they are defined for task-related' dialogues and arguments. For example, in arguments, one DS purpose (DSP) dominates another if the second provides evidence for a point made in the first. When the dominated DSP is satisfied, its corresponding FS is popped. This stack mechanism models the listener's attentional state. The relations between DSPs constitute the intentional structure of the text. Getting a listener to resume a DS via the stack mechanism is taken to require less effort on a speaker's part than returning to elaborate an argument or subtask description later on.</Paragraph>
      <Paragraph position="4"> The significance of Sidner (1983) and Grosz and Sidner (1986) for the current enterprise is that: Computational Linguistics, Volume 14, Number 2, June 1988 63 Bonnie Lynn Webber Tense as Discourse Anaphor * Sidner essentially shows how DF can move gradually among the discourse entities that make up a focus space, as the listener is processing its associated discourse segment; * Grosz and Sidner show how DF can make a radical jump to a different (possibly newly evoked) discourse entity as the listener moves to process the next discourse segment. ~o I reinterpret this in the current framework in terms of the anaphoric function a(NPb,Ea). Within a discourse segment, the entity that is the DF is the most likely E a. Over the discourse segment, other discourse entities in the segment's focus space may in turn become DF.</Paragraph>
      <Paragraph position="5"> With a change in discourse segment, however, the DF can change radically to an entity in the focus space associated with the new segment.</Paragraph>
      <Paragraph position="6"> To hint again at what is to come: in Section 3.2, I will propose a temporal analogue of DF, which I have called temporal focus (TF). In Section 3.3, I will show how gradual movements of the TF are tied in with the ontology of what a tensed clause specifies--i.e., an ontology of events and situations--while more radical movements reflect the effect of discourse structure on TF.</Paragraph>
    </Section>
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