Mental Models Theory and Anaphora

Mental Models Theory and Anaphora Guido Boella and Leonardo Lesmo Dipartimento di Informatica and Centro di Scienza Cognitiva Universit`a di Torino email: guido, lesmo @di.unito. it Abstract We argue that anaphora cannot be resolved at the level of the formal language representing meaning, but, rather, by making direct reference to the extension of the sen- tences. Johnson-Laird’s mental models theory provide the tool for coping with extensional representations in a cognitively plausible way. Introduction Anaphoric expressions are traditionally viewed as sub- stitutes for more complex linguistic expressions which have already occurred earlier in the text. Anaphora has proven difficult to analyze at a purely syntactic level, so that structural approaches like DRT [10] or semantic ones like Dynamic Semantics [4] cope with this problem by enriching the formal language used to build or to rep- resent the meaning of sentences. We believe that the limit of these approaches is that they have chosen the wrong level of representation for dealing with anaphora: we will show that it is necessary to make direct reference to extensional representations of meaning. In particular, the representation of the con- text should put at disposal the elements of the situation, which anaphors can refer to, instead of hiding them be- hind quantified expressions. However, extensions can possibly be infinite or too large to be dealt with directly. But there is a proposal which uses extensional representations of finite and lim- ited size, and which has been shown to be cognitively plausible, i.e., the mental models theory of [9]. Johnson- Laird has used mental models in order to explain how people reason without having to resort to formal logic. Inferences are performed by manipulating extensional representations of sentences which are composed of a fi- nite number of elements and relations: “a mental model represents the extension of an assertion, i.e., the situation it describes, and the recursive machinery for revising the model represents the intension of the assertion, i.e., the set of all possible situations it describes. ” (p.100) In [8]’s words: “mental models theory is a psycholog- ical theory of language processing and reasoning. The theory provides a framework within which more detailed accounts of the component processes of comprehension [. ] such as anaphora interpretation [. ] and reasoning can be developed, [. ] Mental models theory assumes that comprehension results in the construction of repre- sentations of situations in the real world [. ] These mod- els are finite and computable, and they are constructed incrementally, with the model so far acting as part of the context for interpreting the current text. (p.20) A simple preliminary example illustrates our solution. In the following sentences, the acceptability is guaran- teed just for the pair where the (intended) antecedent (a donkey) and the pronoun (they) do not agree in their syn- tactic number: (1) Every farmer owns a donkey. *It is pink. (2) Every farmer owns a donkey. They are pink. When the second sentence in each discourse is inter- preted, it produces a mental model which must be integrated with the preceding one: a referent must be found for the anaphoric expressions. If we ex- amine in the Figure below how the first sentences of the two pairs are represented in mental models theory, we see that the problem is easily solved. The mental model contains own f d a finite num- own f d ber of tokens own (placeholders f d for individuals, here farmers and donkeys ) and relations among tokens (the arrows labeled with own). Given the model above, which donkey, out of the represented ones, can we relate to the singular it, appearing in the second sentence of (1)? The problem of identifying the referent appears to be the same as in: (3) I have three sisters. *She is blonde where we have to choose one referent out of three candidates. One is given no (or not enough) information to identify the antecedent (among the three sisters) denoted by she. On the contrary, the they pronoun in (2) can be interpreted as referring to the set of donkeys appearing in the model, due to its plural syntactic number. The mental model building algorithm First of all, the sentence undergoes a syntactic and se- mantic interpretation process that produces a semantic network (see [6], [11] and [2] for details on the network representation). Then, following the proposal by [9], that “a propositional representation can be used as the input to a procedural semantics that constructs mental mod-