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<Paper uid="C90-2017">
  <Title>Discourse Anaphora</Title>
  <Section position="4" start_page="95" end_page="96" type="intro">
    <SectionTitle>
4 Motivation
</SectionTitle>
    <Paragraph position="0"> The approach claims that anaphoric resolution is a two-stage procedure. The first stage determines the possible antecedents by means of tile linguistically defined notions of compatibility and recency. The second stage consists of the calculation of preferred antecedents. I will first argue why the distinction between possible and preferred is important. And second, \[ will motivate why the definition of possible antecedents is expressed in terms of the notions compatibility and recency.</Paragraph>
    <Paragraph position="1"> The distinction possible vs. preferred is considered essential for the following reasons. First, it is a means to determine whether a discourse is coherent or not, from the anaphoric point of view. Consider what happens when an antecedent is selected but needs to be rejected because of information later in the discourse. As the meanings of terms like possible/impossible suggest, there is no way that we can backtrack on an initially possible antecedent in favour of an antecedent that is not part of the set of possible ones. Compare the following two discourses: (1) John was late for his appointment with Joe Fortunately, Joe was even later ttis work had kept him from leaving in time He, on the other hand, had missed the bus  (2) John was late for his appointment with Mary Fortunately, Mary was even later His work had kept him from leaving in time She, on the other hand, had missed the bus Discourse (1)is clearly incoherent whereas (2) is acceptable. The reasons are purely linguistic. The definition of possible antecedents above stated that some antecedents are not accessible to a pronoun because of other intermediate compatible antecedents. Only the latter are possible candidates. When the continuation of the discourse makes clear that they were not the right antecedents after all, the discourse should be considered incoherent. A preferred antecedent, however, may be rejected later on in the discourse in favour of one that was not preferred, merely possible. An algorithm that merges possible and preferred is not able to make this distinction. null The second motivation for maintaining the possible/preferred distinction is provided by (linguistic) ambiguity in the language. Consider (3) and the examples cited in Winograd(1972), (4) vs.</Paragraph>
    <Paragraph position="2">  (3) The women met their husbands at a party They were very young at the time (4) The city councillors refused the demonstrators a permit because they feared violence (5) The city councillors refused the demonstra null tors a permit beca.use they advocated violence null Discourse (3) is three-way ambiguous and it is vague which of the meanings is intended. Blurring the distinction between preferred and possible antecedents right away contradicts the intuition that all three antecedents in the first sentence might serve as antecedents for the pronoun but not any NP that was mentioned before this utterance. The examples (4) and (5) serve to show that linguistically possible is essentially different from possible as regards 'the world'. Assuming that no NLP-system, nor any human discourse participant by the way, has rich enough information to infer all essentials at the right time (yet), this approach at least yields a reliable, and finite, set of possible candidates. Having shown that the distinction between well-formed and preferred antecedent-anaphor relations is necessary, it remains to be shown why possible antecedents are defined by means of the simple notions of compatibility and recency. The explanation is quite simple: they seern to yield the right results. It is well-known that full deftnite NPs are in general further apart from their antecedents than pronouns are. Now consider that definite NPs, in general again, have more descriptive content than pronouns do. It follows that pronouns may be compatible with antecedents that definite NPs do not match with.</Paragraph>
    <Paragraph position="3"> Together with the notion of recency, compatibility then explains why the antecedents of definite NPs may be at quite a distance. This only holds for definite NPs that have ample descriptive content though. An underspecified definite NP like 'the man' behaves similarly to the pronoun 'he'. Both will accept the most recently mentioned male individual(s) as their possible antecedent(s). So, the number of units that the anaphor may search to find its antecedent is dynamic rather than static. Consider discourse (1) versus (2) again. A static number of sentences would not explain the difference. The pronouns in (1) behave similar to those in (2) and yet, one of the two is incoherent. The reason is that 'John' in (1) is not accessible because of the intervening NIP 'Joe'. The NP 'Mary' in (2) does not block reference to 'John' because 'Mary' is not compatible with the same anaphor.</Paragraph>
  </Section>
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