Comparing Two Views of Metaphor Semantics Carl Vogel? and Josef van Genabithy Trinity College? University of Dublin Computer Science Dublin 2, Ireland
[email protected]
Dublin City Universityy Computer Applications Dublin 9, Ireland
[email protected] Abstract Criteria are presented and argued as necessary qualities of any formal semantic account of the meaning of metaphorical sentences in natural language. Two recent frameworks that attempt to provide semantic analyses for metaphor are critiqued relative to these criteria.
1 Introduction One line of thought about metaphor is that it is a defective form of language use. Certainly, it has been neglected by most researchers in natural language semantics working within a model theoretic tradition. This is in part because by its very nature as nonliteral language, metaphorical sentences are hard to accommodate in any model of sentence meaning that gives prominence to truth conditions. Davidson (1984) has gone so far as to suggest that metaphoricity isn’t a property of sentences themselves, but uses of sentences, something that takes them outside the field of study for semanticists. However, there has been a recent approach in semantics that gives equal attention to the change of information state induced by the interpretation of a sentence (hence its impact on the interpretation of subsequent sentences). Such an approach makes it possible to address some aspects of language use in the formal semantic models. There are two approaches that we know of that attempt to offer a compositional semantics for metaphor. The first (Vogel, 1998, 1999) assumes translation from natural language to a formal logical language. Vogel (1998) uses translation to first order logic and Vogel (1999) uses a more expressive intensional logic. The logical languages are given nonclassical interpretations, however; a dynamic system is used so that a nonliteral expression can have the impact on subsequent discourse that though literally false, it is nonliterally true. The second approach is offered by van Genabith (1999). This system uses a fully classical,
but expressive type theoretic framework. In this work, each metaphorical sentence is read as corresponding reduced simile. A goal of this paper is to critique these two approaches. We outline a number of features that should be present in a good model theoretic semantics for metaphorical sentences. We provide an overview of each of the two main approaches in this area, evaluating them with respect to the desiderata. Finally, we compare the theories directly with each other.
2 Desiderata 2.1 Compositionality On the one hand, it is often assumed that metaphor presents a prima facie case for semantics to deal with a sentence whose meaning is more than the sum of its parts, on the other hand, it is argued that a metaphor is simply not effectively paraphrased by a related simile. Davidson (1984) makes this point quite clearly.1 Both the elliptical simile theory of metaphor and its more sophisticated variant, which equates the figurative meaning of the metaphor with the literal meaning of a simile, share a fatal defect. They make the hidden meaning of the metaphor all too obvious and accessible. In each case the hidden meaning is to be found simply by looking to the literal meaning of what is usually a painfully 1 Pg.
254.
trivial simile. This like that—Tolstoy is like an infant, the earth like a floor. It is trivial because everything is like everything, and in endless ways. Metaphors are often very difficult to interpret and, so it is said, impossible to paraphrase.
It is important to realize that Davidson’s criticism only applies if a corresponding simile is allowed to range into the trivial. It is more than doubtful, however, that in most cases of actual use similes are used trivially in communication. In addition, it remains to address the question of what that “hidden” meaning that is more than the sum of the parts could be. A theory of metaphor that provides an answer should at least capture the fact that there is not just any arbitrary likeness between the vehicle and tenor that a simile seems to explicitly existentially quantify away. The metaphorical meaning must be given a clear model just as literal predicates are modeled by their characteristic sets. Moreover, this meaning must emerge from the composition of the parts of a sentence in a context in a way that discriminates between metaphorical and literal readings.
2.2 Truth Conditions Another way of stating the above constraint on theories of metaphor is that truth conditions remain important to our understanding of language. On the face of it, metaphors and similes have very different truth conditions, and this difference should be localized in any theory of metaphor. Similes cannot be false; they assert the existence of a likeness that does not have to be pinned to any particular property (1). (1)
The nation is like a cheesecake. It follows that the negation of a simile is false:
(2)
The nation is not like a cheesecake.
2.3 Aptness The aptness of a metaphorical sentence is something like a measure of its appropriateness in a given context. However, it has a different nature than choice of speech act or other pragmatic decisions that occur in the formulation of sentences to determine appropriateness. (6)
Could you please pass the salt?
(7)
This soup could do with a bit more salt.
(8)
This soup was not made with the waters of the Great Salt Lake.
This is partly evident from the fact that speech acts combine with metaphors. The difference to finding the right level of politeness is that aptness is a measure of semantic appropriateness, even though it does draw on world knowledge, and context in addition to the sentence at hand. We use the expression “semantic appropriateness” to address the intuition that while literally the sentence seems false (or true, depending on the negative polarity), relative to the nonliteral senses at stake, the sentence is true. In fact, the primary issue at stake with aptness is that although the grounds need to be relativized to nonliteral senses, truth does seem to be the right way of characterizing the basic level of aptness of a metaphor. Of course, this is not all there is to aptness. Some are more apt than others. This seems to involve ancillary predications, and the degree to which the metaphor “extends” or falls apart under scrutiny. The contribution of these additional predications is still in terms of their holding or not, but with some further assessment of their number, and as well of their analogical importance to the metaphor at hand, where analogical importance can be determined structurally using mappings between associated implicative complexes (Veale and Keane, 1992).2
Notice, however that this observation only holds if we are prepared to admit trivial likeness as in Davidson’s “. . . everything is like everything, and in endless ways.” Trivial use of similes of this sort is probably quite rare in actual communicative exchanges (except in puns and jokes). Rather, as is the case with metaphor, the communicative function of a simile is an instruction for the recipient to establish some particular properties instantiating likeness between tenor and vehicle. By contrast, metaphors seem to be literally false (3), and their negations true (4).
2.4 Novelty
(3)
The whale is a blimp.
(4)
The whale is not a blimp.
It is an accepted metaphorical sense of (10) that allows the sentence to be used to express that Leslie is clever.
(5)
No man is an island
Yet, sentences like (5) demonstrate that metaphoricity transcends falsity. It isn’t the case that every false sentence is metaphorical.
Clearly metaphoricity depends on language use rather than timeless properties of sentences. There is a difference between the first use of a new metaphor and the re-use of an established one. First uses are rather more difficult to process as they involve essentially rendering another sense for an existing predication. (9)
Leslie washes books.
(10) Leslie is a fox.
2 By “implicative complex” we mean just the set of sentences that pertain to some expression — the set of sentences implied by an expression (when it’s a sentence on its own), the set of sentences that imply it, etc.
However, in a context in which (9) is literally false, additional information is required to make the sentence interpretable. There are any number of scenarios that render the sentence metaphorically apt, one of which is that Leslie copy edits manuscripts of books. Once that extension is made, it is readily possible to re-use that sense. The first use can be quite difficult to obtain, and it is claimed that there are syntactic constraints on where new metaphors can be constructed that are distinct from constraints on re-use of existing metaphors (Vogel, 1998). (11) Sal washes newspaper articles. Moreover, it is necessary to account for the fact that eventually, novel metaphors become so entrenched in everyday use that they cannot be discriminated from expressions whose interpretation is intended to be literal. By now “fox” as nonliterally used in (10) is close to being a dead metaphor. Dead metaphors are ubiquitous in language. It is necessary to have a succinct model of the potential for transition from novel use to accepted metaphor to dead metaphor.
2.5 Semantics-Semantics Interface Important to a theory which satisfies the above constraints is that it integrate the interpretation of metaphor and other nonliteral language (whose context change potential and truth conditions are of primary interest) with the interpretation of literal language. While the different uses of language can be discriminated in some sense within an overarching system. It is inappropriate to have one mechanism for literal language and an altogether different module for dealing with nonliteral language. The reason for this is suggested by the preceding section: it has often been demonstrated that language coordination isn’t often by explicit negotiation but by use and mimicry (Garrod and Anderson, 1987; Healey, 1995). This (and popular perceptions that prescriptive linguistic institutions ultimately fail) suggests that the primary mechanisms for literal language ontogenesis are closely tied to the creation and death of metaphor. If literal language springs from the nonliteral, then it is the literal that is “deviant” if either is. “Deviance” is to be the wrong mode of expression here: it is more satisfying to have a single framework in which both literal and nonliteral language are handled equally but discriminated in the important senses than to set up systems in which semantics is done on literal language and only some other processing on nonliteral language.
3 A Critique of Recent Proposals We contrast two recent proposals for integrated semantic analysis of literal and nonliteral sentences in formal semantics.
3.1 Intensional Dynamic Semantics 3.1.1 Overview Vogel (1999) presents an intentional predicate calculus as a meaning representation language for both literal and nonliteral expressions. The language has greater than first order expressivity in that it admits predicate polysemy, where each sense of a predicate corresponds to a corresponding word sense. This is a very weak extension of first order logic, though, as it amounts to being able to index predicates to enumerate their meanings ( BANK1 is a financial institution, BANK2 is the effluvia of a body of water, ....). It is a real extension, nonetheless, as it requires a more complex semantics that provides an index for the characteristic set of individuals satisfying the predicate in each of its senses. Some indices are classified as literal, and others are classified as nonliteral. Indices can be referred to directly within the language, but reference to indices isn’t essential for syntactic or semantic well-formedness. Underspecification of indices simply results in ambiguity. New indices can be created, and new tuples can be added to their characteristic sets. Vogel (1999) provides one set of constraints on this process that restricts where characteristics can be extended. Once a characteristic set is extended, the newly resulting model provides the context of interpretation for subsequent discourse. Thus there are two ways in which a classical predicate logic is extended. The first is to address polysemy. The approach here is to make the system weakly intensional—direct reference to indices is possible in the language but not necessary; no constraints are placed on the multiplicity of senses. The second issue is that the system is given dynamic interpretation. The interpretation function must be examined both before and after the interpretation of sentences. Some sentences have the capacity to extend the function, and this extension is passed on to the interpretation of subsequent discourse. 3.1.2 Critique Compositionality. As presented by Vogel (1999), this system treats metaphor as a mode of expression distinct from simile. Metaphoricity comes from distinguishing the sense of the predicate. In a way, this reduces metaphoricity to polysemy with a preference ordering on senses of expressions: new uses are most metaphorical, nonliteral but old ones admit new tuples into their remit, old uses are deemed literal. The system presumes a translation function that maps natural language sentences into the very rich meaning representation language and at the point of translation introduces ambiguity: no stipulations are given on how parts of natural language syntax guide the interpretation into sense selecting expressions or deixis. However, once translated, the sentences is fully distinct from related simile. The “hidden” meaning of the metaphor is just the pointer to the particular sense at stake. In this framework, metaphorical meaning is thus modeled exten-
sionally. Truth Conditions. In this system a predicate can be false relative to one sense and true relative to another, whether those senses are classified as literal or nonliteral. Whether the exact truth conditions proposed are correct is subject for debate. Aptness. The theory says nothing about aptness, apart from supplying a way for the metaphorical sentence to be nonliterally true, and reducing that truth to extensionality. What the system lacks is a way of clarifying what other predicates require extension, given that one has extended one. It is clear that the sort of mechanism that would work in this context is one that examines the web of implications that a predicate participates in, and on the basis of structural analogy maps those predicates to predicates corresponding to the analogical domain. More has to be said about this interface, but it is clear that this exactly the sort of theory that has been worked out by Veale and Keane (1992) in the context of network encodings rather than first order encodings. Novelty. Ontogenetic uses of metaphor are distinguished from re-uses of existing senses for predicates. Shift of senses is modeled by reclassification of indices as literal or nonliteral, however no constraints are placed on this mechanism besides the basic classification. The precise syntactic (predicate form) and semantic (literal falsity of the predicate) constraints on first use of a metaphor generate empirical claims that require testing. Semantics-Semantics Interface. Because all senses of expressions are extensionalized as characteristic sets of predicates at indices, literal and nonliteral expressions are handled within the same overarching framework, with a difference in precise function corresponding to the classification of indices at stake as literal or nonliteral.
3.2 Type Theoretic Semantics 3.2.1 Overview In van Genabith (1999) a standard type theoretic approach is employed to provide an analysis of metaphors as reduced similes (cf. Davidson (1984) and discussion above). On this account (10) is translated as Leslie and the set of all foxes share a common property. Sense extension is accounted for indirectly in that the common property includes at least the extension of Leslie and the set of all foxes. However, this simple-minded approach is easily trivialized by the universal property (the property of being identical to oneself). The translation is strengthened by requiring that the property sought be non-trivial. The type theoretic representation is obtained compositionally (Montague, 1973). The approach captures both direct copula constructions and more complex metaphors
involving other predicates (such as sentence (9)). A Prolog implementation is provided. 3.2.2 Critique Compositionality. Compositionality is one of the motivating factors for this approach. van Genabith (1999) provides a syntactic fragment and corresponding semantic interpretations for each rule in the fragment. Each sentence generated by the fragment is ambiguous between a literal and a metaphorical translation which interprets a metaphor as a corresponding, reduced simile, as discussed above. Truth Conditions. The truth conditions are classical. The system derives its power from the translation. Given a metaphor, the corresponding simile translations augmented with a non-triviality constraint come out as contingent (either true or false). Aptness. Like the approach of Vogel (1999), this framework relies on the existing literature on metaphor recognition to determine what counts as a good metaphor. The approach offers an initial idea towards simile property instantiation (i.e. property resolution). It remains to be seen whether this scales up to more complex examples. Novelty. This theory was formulated with compositionality and interface to other semantic modules as its primary concern. It does not address issues of fossilization. The system is like classical logic in being completely static. The re-use of metaphor isn’t distinguished from first use, but is distinguished from literal language. Semantics-Semantics Interface. The formulation offered by van Genabith (1999) satisfies this desideratum because that was it motivating concern. The same mechanisms underly the literal and nonliteral interpretation of sentences. What discriminates the two interpretations is precisely the syntax semantics interface—the choice of translation rules.
4 Discussion This abstract offers a set of criteria for a semantic theory for metaphorical uses of sentences. We have described two theories that offer compositional denotational semantics to metaphors, something that other theories of metaphor interpretation lack.
Acknowledgements Many thanks to Julia Hockenmaier for last minute assistance.
References Donald Davidson. What metaphors mean. In Donald Davidson, editor, Inquiries into Truth and Interpretation, pages 245–64. Oxford: Oxford University Press, 1984. Simon C. Garrod and Anthony Anderson. Saying what you mean in dialogue: A study in conceptual and semantic co-ordination. Cognition, 27:181–218, 1987. Pat Healey. Communication as a Special Case of Misunderstanding: Semantic Coordination in Dialogue. PhD thesis, Centre for Cognitive Science, University of Edinburgh, 1995. R. Montague. The proper treatment of quantification in ordinary english. In J. Hintikka e.a., editor, Approaches to Natural Language, pages 221–242. Reidel, 1973. Josef van Genabith. Metaphor and type theory. Symposium on Metaphor, Artificial Intelligence and Cognition, Society for Artificial Intelligence and Simulation of Behavior, April 6-9., 1999. Tony Veale and Mark Keane. Conceptual scaffolding: A spatially founded meaning representation for metaphor comprehension. Computational Intelligence, 8(3):494– 519, 1992. Carl Vogel. A dynamic semantics for novel metaphor. Third International Conference on Information Theoretic Methods in Logic, Language and Computation, Hsi-Tou, Taiwan, June 16-19., 1998. Carl Vogel. An intensional dynamic semantics for polysemy, sense extension and metaphor. Symposium on Metaphor, Artificial Intelligence and Cognition, Society for Artificial Intelligence and Simulation of Behavior, April 6-9, 1999.
