Metaphor Comprehension : From Comparison to Categorization Brian F. Bowdle Dedre Gentner Department of Psychology Department of Psychology Indiana University Northwestern University 1101 East 10' Street 2029 Sheridan Road Bloomington, IN 47405 Evanston, IL 60208 (bbowdle@indiana .edu) (gentner@nwu .edu) Abstract tony, 1979 ; Tversky, 1977) . However, more recent ver- sions of the comparison view have assumed that metaphors In this paper, we explore the relationship between metaphor act to set up correspondences between partially isomorphic and polysemy . We begin by discussing how novel meta- conceptual structures rather than between sets of independ- phoric mappings can create new word meanings in the form ent properties (e .g., Gentner, 1983 of domain-general representations. Turning next to consider ; Indurkhya, 1987 ; Kit- the implications of this view for the on-line comprehension of tay & Lehrer, 1981 ; Lakoff & Johnson, 1980 ; Verbrugge figurative language, we suggest that there is a shift from & McCarrell, 1977) . In other words, metaphor can be seen comparison processing to categorization processing as meta- as a species of analogy, phors are conventionalized . Finally, we describe a series of We will use Gentner's (1983) structure-mapping theory experimental findings that support the proposed account. to articulate the processes that may take place during meta- phor comprehension . Structure-mapping theory assumes that interpreting a metaphor involves two interrelated Introduction mechanisms: alignment and projection . The alignment process operates in a local-to-global fashion to create a Metaphors establish mappings between concepts from dis- maximal structurally consistent match between two repre- parate domains of knowledge . For example, in the meta- sentations that observes one-to-one mapping and phor The mind is a computer, an abstract entity is described parallel connectivity (Falkenhainer, Forbus, & Gentner, 1989) . That in terms of a complex electronic device . It is widely be- lieved that metaphors are a major source of knowledge is, each object of one representation can be placed in corre- change, and a great deal of research has examined how spondence with at most one object of the other representa- metaphors can enrich and illuminate concepts that would tion, and arguments of aligned relations are themselves aligned. A further constraint on the alignment process is otherwise remain vague or ambiguous . However, there systematicity : Alignments that form deeply interconnected have been far fewer . explorations of a second generative structures, in which higher-order relations constrain lower- function of metaphors - namely, lexical extension . In this order relations, are preferred over less systematic sets of paper, we will discuss (1) how novel metaphoric mappings commonalities. Once a structurally consistent match be- can create new word meanings in the form of domain- general representations, and (2) how these new meanings tween the target and base domains has been found, further predicates from the base that are connected to the common may be applied during the comprehension of conventional system can be projected to the target as candidate infer- metaphors . Before turning to these issues, however, it is ences . necessary to consider the nature of metaphoric mappings in According to structure-mapping theory, metaphors of- greater depth . ten convey that a system of relations holding among the base objects also holds among the target objects, regardless Metaphor and Analogy of whether the objects themselves are intrinsically similar. Metaphors are traditionally viewed as comparisons between Thus, the metaphor Socrates was a midwife highlights cer . the target (a-term) and the base (b-term) . According to tain relational similarities between the individuals - both many early models, metaphors are understood by means of help others produce something - despite the fact that the a simple feature-matching process (e.g., Miller, 1979 ; Or- arguments of these relations are quite different in the target Bowdle, B . F., & Gentner, D . (1999) . Metaphor comprehension: From comparison to categorization. Proceedings of the Twenty-First Annual Conference of the Cognitive Science Society, 90- 95 . 90 and base domains: Socrates helped his students produce same base term is repeatedly aligned with different targets ideas, whereas a midwife helps a mother produce a baby. so as to yield the same basic interpretation, then the high- The centrality of relations during metaphor comprehension lighted system may become conventionally associated with has been confirmed by a number of studies . For example, the base as an abstract metaphoric category. At this point, people's interpretations of metaphors tend to include more the base term will be polysemous, having both a domain- relations than simple attributes, even for statements that specific meaning and a related domain-general meaning . suggest both types of commonalities (e.g., Gentner & Clem- We will refer to this proposed evolution as the career of ent, 1988; Shen, 1992 ; Tourangeau & Rips, 1991) . Fur- metaphor hypothesis . (For related proposals, see Holyoak ther, Gentner & Clement (1988) found that the relationality & Thagard, 1995 ; Murphy, 1996) . of people's interpretations of metaphors was positively re- lated to the judged aptness of these same metaphors . Implications for Metaphor Comprehension Research on metaphor comprehension often treats metaphor Metaphor and Polysemy as an undifferentiated type of figurative language . How- Like analogies, metaphors can lend additional structure to ever, a number of theorists have recently argued that meta- problematic target concepts, thereby making these concepts phor is pluralistic, and that the manner in which a metaphor more coherent. However, this is not the only way in which is comprehended may depend on its level of conventionality metaphors can lead to knowledge change . Metaphors are (e.g., Blank, 1988; Blasko & Connine, 1993 ; Giora, 1997 ; also a primary source of polysemy - they allow words with Turner & Katz, 1997). Our account of the relationship be- specific meanings to take on additional, related meanings tween metaphor and polysemy is in line with these claims . (e.g., Lakoff, 1987 ; Lehrer, 1990 ; Miller, 1979; Nunberg, Specifically, we believe (1) that the process of convention- 1979; Sweetser, 1990) . For example, consider the word alization is essentially one of a base term acquiring a do- roadblock. There was presumably a time when this word main-general meaning, and (2) that this representational referred only, to a barricade set up in a road . With repeated shift will be accompanied by a shift in mode of processing . metaphoric use, however, roadblock has also come to refer These ideas are illustrated in Figure 1, which shows to any obstacle to meeting a goal (as in Fear is a roadblock how novel and conventional metaphors differ on the career to success). of metaphor view . Novel metaphors involve base terms that How do metaphors create new word meanings? One refer to a domain-specific concept, but are not (yet) associ- recent and influential proposal is that such lexical exten- ated with a domain-general category . For example, the sions are due to stable projections of conceptual structures novel base term glacier (as in Science is a glacier) has a and corresponding vocabulary items from one (typically literal sense - "a large body of ice spreading outward over a concrete) domain of experience to another (typically ab- land surface" - but no related metaphoric sense (e.g., stract) domain of experience (e .g., Lakoff, 1987 ; Lehrer, "anything that progresses slowly but steadily") . Novel 1990; Sweetser, 1990). On this view, the metaphoric metaphors are therefore interpreted as comparisons, in meaning of a polysemous word is understood directly in which the target concept is structurally aligned with the lit- terms of the word's literal meaning . eral base concept. However, metaphoric categories may We wish to consider an alternative account of the rela- arise as a byproduct of this comparison process . tionship between metaphor and polysemy - one that follows In contrast to novel metaphors, conventional metaphors naturally from viewing metaphor as a species of analogy . involve base terms that refer both to a literal concept and to Research on analogical problem solving has shown that the an associated metaphoric category . For example, the con- alignment of two relationally similar situations can lead to ventional base term blueprint (as in A gene is a blueprint) the induction of domain-general problem schemas that can has two closely related senses: "a blue and white photo- be applied to future situations (e.g., Gick & Holyoak, 1983 ; graphic print in showing an architect's plan" and "anything Novick & Holyoak, 1991 ; Ross & Kennedy, 1990) . We that provides a plan." Conventional base terms are polyse- believe that similar forces are at work during metaphor mous, and the literal and metaphoric meanings are semanti- comprehension . The central idea is that the process of cally linked due to their similarity. Conventional metaphors structural alignment allows for the induction of metaphoric may therefore be interpreted either as comparisons, by categories, which may in turn be lexicalized as secondary matching the target concept with the literal base concept, or senses of metaphor base terms (Bowdle, 1998 ; Bowdle & as categorizations, by seeing the target concept as a member Gentner,