Three moDels

moDel ONE: Recency Constraint (Hobbes 78)

First Priority to the most recently mentioned Discourse Entity
1) John helped Mary.
2) He was kind.                    // "He" refers to Mary??
Therefore, we need to add Constraints:
1) Number
2) Gender
3) Reflexivity
Yet, this is still an overly Simplistic model, as illustrated below.
1) John met Peter.
2) He was kind.                // "He" refers to Peter??

moDel Two: Centering Theory (Grosz et al. 95)

Idea: Focus of attention and Local coherence - inference load upon hearer.
Center means the discourse focus, which is typically pronominalized.
Structures include Forward Looking Centers & a Backward Looking Center.

Ranking = Subject >> Object >> others

Two Governing Rules
1) The center must be pronominalized if other DEs are
2) Continuation with the same center is preferred

moDel three: S-List Model (Strube 98)

This model extends from Centering Theory and claims to perform better. It has a single construct and the ranking is as follows:
      Hearer-old >> Mediated >> Hearer-new
1) hearer old – evoked
2) mediated – inferrables
3) hearer-new – brand new

 Cases
1) same class, different utterance: we favor the pronoun in the later utterance
2) same class, same utterance: we favor the pronoun in the earlier position