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<Paper uid="P98-2249">
  <Title>A Cognitive Model of Coherence-Driven Story Comprehension</Title>
  <Section position="3" start_page="1499" end_page="1499" type="intro">
    <SectionTitle>
2 Coherence and Satisficing
</SectionTitle>
    <Paragraph position="0"> A schema is any function which maps inputs onto mental representations. It contains slots which can be instantiated using explicit input statements, or implicit statements derived via proof or assumption. Instantiated schemas form the building blocks of the comprehender's representation. A comprehender has available both 'weak' schemas, which locally link small amounts of input (e.g. causal schemas); and 'strong' schemas, which globally link larger sections of input (e.g. scripts).</Paragraph>
    <Paragraph position="1"> All schemas generate 'connections of intelligibility' which affect the coherence of a representation (Harman, 1986). Coherence is a common 'currency' with which to measure the benefit of applying a schema. Instead of requiring that a top-level structure be instantiated, the system instead applies schemas to produce a representation of sufficient 'value'. This process can be naturally described as abduction, or 'inference to the best explanation' (Ng and Mooney, 1990).</Paragraph>
    <Paragraph position="2"> Previous natural-language abduction systems can form more-or-less coherent representations: for example, by halting comprehension when assumptions start to reduce coherence (ibid.).</Paragraph>
    <Paragraph position="3"> However, these systems still have a fixed 'cutoff' point: there is no way to change the criteria for a good representation, for example, by requiring high coherence, even if this means making poorly-supported assumptions. By treating coherence as the currency of comprehension, the emphasis shifts from creating a 'complete' representation, to creating a satisficing one. (A satisficing representation is not necessarily optimal, but one which satisfies some minimal constraint: in this case, a coherence threshold.)</Paragraph>
  </Section>
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