Local Discourse and Reference

Reference: Chapter 14 of Allen
Aim:
This section concerns the problem of deciding what phrases (especially noun phrases) refer to. It introduces a simple model of global discourse structure called the history list, and presents an algorithm for referent determination in simple cases.
Keywords: anaphor, antecedent, cataphor, co-refer, DE, DE list, discourse entity, discourse entity list, evoke, history list, local discourse context, reference, referential ambiguity
Plan:
  • anaphors
  • world models
  • discourse entities, DE list, evoked DEs
  • creation of DEs
  • quantifier problems
  • history list, simple reference algorithm

Anaphoric Reference

A key concept in reference is that of an anaphor.

anaphora: The explication of a word by words appearing previous to it in the text: Mary died. She was very old. (She is explained by Mary.)
(Macquarie Dictionary)


14.1 Defining Local Discourse Context and Discourse Entities


Local Discourse Context and Discourse Entities 2


Local Discourse Context and Discourse Entities 3


Evoked Discourse Entities


Evoked Discourse Entities 2


Generating Discourse Entities


Indefinite Noun Phrases within the Scope of Universals

picture of a pizza

10aThree boys[i] each bought a pizza[j].
10bThey[i] ate them[j] in the park.

Initial translation of 10a:

  ∃ B : |B| = 3 & {x | Boy(x)} ⊇ B
picture of a pizza       ∀ b : b ∈ B .
          ∃ p : Pizza(p) . Buy(b, p)

Discourse entities from 10a:

   B1: |B1| = 3 & {x | Boy(x)} ⊇ B1
picture of a pizza    P1: P1 = {x | Pizza(x) & ∃ y ∈ B1 . x = sk4(y)}

Semantic content:

   ∀ b : b ∈ B1. sk4(b) ∈ P1 & Buy1(b, sk4(b))

This is related to the ambiguity issue we considered with the sentence Every man loves a woman in the section on logical form.


14.2 A Simple Model of Anaphora Based on History Lists


Antecedent-Finding Algorithm

  1. check the most recent local context for an antecedent that matches all the constraints related to the pronoun.

    Constraints include number, gender, and other grammatical phenomena such as the rules for the use of reflexive pronouns.

    Semantic constraints imposing selectional restrictions may occur too: e.g. in There was cheese on the table. Jack ate it. the word it cannot refer to the table.

  2. if no referent is found in the current local context, move down the history list to the next most recent local context and search there.


Definite Descriptions


Indirect Reference

 

 


Indirect Reference 2

20aMy club held a raffle.
20bThe winner won a car.

Indirect Reference 3

22aJack brought a pencil to class.
22bHowever, he found that the lead was broken.

Summary: Local Discourse and Reference
Concentrating mostly on anaphoric reference to noun phrases, we have described a simple algorithm for deciding what object in the history list a phrase in the text refers to. The key ideas presented include searching the history list in most-recent-first order, using relationships between objects to determine indirect references. We also looked briefly at the effects of the use of quantifiers like "each" on the referent of an object within the scope of the quantifier.

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Copyright (C) Bill Wilson, 2007, except where another source is acknowledged. Most material is adapted from James Allen's book "Natural Language Understanding".