Formalizing the Pragmatics of Metaphor Understanding

Formalizing the Pragmatics of Metaphor Understanding Justine T. Kao 1 (justinek@stanford.edu) , Leon Bergen 2 (bergen@mit.edu) , Noah D. Goodman 1 (ngoodman@stanford.edu) 1 Department of Psychology, Stanford University. 2 Department of Brain and Cognitive Science, MIT. Abstract theorists argue that this principle explains how listeners in- fer the meaning of a novel metaphor as well as other forms of loose talk where the meaning of an utterance is underspecified (Sperber & Wilson, 1985). When interpreting the metaphor “My lawyer is a shark,” for example, the listener assumes that the speaker aims to communicate features of “a shark” that are relevant to the person under discussion (“my lawyer”) and do not make use of other shark features—vicious but not has fins or swims. While the ubiquity and importance of nonliteral language are clear, people’s ability to use and understand it remains a mys- tery. Metaphor in particular has been studied extensively across many disciplines in cognitive science. One approach fo- cuses on the pragmatic principles that listeners utilize to infer meaning from metaphorical utterances. While this approach has generated a number of insights about how people under- stand metaphor, to our knowledge there is no formal model showing that effects in metaphor understanding can arise from basic principles of communication. Building upon recent ad- vances in formal models of pragmatics, we describe a com- putational model that uses pragmatic reasoning to interpret metaphorical utterances. We conduct behavioral experiments to evaluate the model’s performance and show that our model produces metaphorical interpretations that closely fit behav- ioral data. We discuss implications of the model for metaphor understanding, principles of communication, and formal mod- els of language understanding. While many linguists and psychologists have argued for the benefits of studying metaphor using a pragmatics frame- work, to our knowledge there is no formal model showing that effects in metaphor understanding may arise from basic principles of communication. On the other hand, a recent body of work presents a series of computational models for pragmatic reasoning, where speaker and listener reason about each other to communicate effectively (Frank & Goodman, 2012; J¨ager & Ebert, 2009). By formalizing principles of communication, these models are able to make quantitative predictions about a range of phenomena in language under- standing, such as scalar implicature and the effect of alter- native utterances (Goodman & Stuhlm¨uller, 2013; Bergen, Goodman, & Levy, 2012). However, a limitation of these models is that they are unable to explain the use of utterances which are known to have false literal interpretations. More re- cent work extends these models to consider affective goals in communication that may be optimally satisfied by nonliteral utterances such as hyperbole (under review). In this paper, we will be considering the role of communicative goals in the in- terpretation of metaphorical utterances. In our formalization, a listener assumes that the speaker chooses an utterance to maximize informativeness about a subject along dimensions that are relevant to the conversation and consistent with the speaker’s communicative goal. This makes it possible for a literally false utterance to be optimal as long as it is informa- tive along the target dimension. Our framework closely aligns with the relevance-theoretic view that a listener considers the relevance of a potential meaning to the speaker’s goal in order to infer what the speaker intended to communicate. Keywords: language understanding; metaphor; pragmatics; computational models Introduction From “Juliet is the sun” to “That woman is a bombshell,” non- literal language is, quite literally, everywhere. Metaphor, hy- perbole, and sarcasm are ubiquitous features of human com- munication, often creating poetic or humorous effects that add rich dimensions to language (Glucksberg, 2001; Pilking- ton, 2000; Lakoff & Turner, 2009; Roberts & Kreuz, 1994). Metaphor has inspired a particularly large amount of research in cognitive science, spanning topics such as how metaphors structure and shape our thoughts (Ortony, 1993; Lakoff et al., 1993; Thibodeau & Boroditsky, 2011), whether metaphor processing recruits the same strategies as standard language processing (Giora, 1997; Gibbs, 2002; Glucksberg & Keysar, 1993) and what factors determine people’s interpretation a novel metaphor (Gentner & Wolff, 1997; Blasko & Connine, 1993; Tourangeau & Sternberg, 1981; Kintsch & Bowles, 2002). This overwhelming interest in metaphor research is due to both the ubiquity of metaphor in everyday language and the potential role of metaphor for helping us understand how the mind creates meaning. One approach to studying metaphor focuses on the prag- matic principles that listeners utilize to infer meaning from metaphorical utterances (Tendahl & Gibbs, 2008). Rather than view metaphor as a separate mode of communication that requires specialized language processing strategies, this approach argues that basic principles of communication drive the meaning that a listener infers from a metaphor (Sperber & Wilson, 2008; Wilson & Carston, 2006). Relevance the- ory, in particular, posits that listeners interpret utterances with the assumption that speakers produced them because they are maximally relevant (Wilson & Sperber, 2002). Relevance To reasonably limit the scope of our work, we focus on metaphors of the classic form “X is a Y . ” We describe a com- putational model that can interpret such sentences metaphor- ically and conduct behavioral experiments to evaluate the model’s performance. We show that peoples’ interpretations of metaphors are driven by conversational context and that our model captures this effect. Finally, we show that our model predictions correlate significantly with people’s fine- grained interpretations of metaphorical utterances.