Computer Understanding of Conventional Metaphoric Language

Metaphor is a conventional and ordinary part of language. An approach to metaphor, based on the explicit representation of knowledge about metaphors, has been developed. This approach asserts that the interpretation of conventional metaphoric language should proceed through the direct application of speci c knowledge about the metaphors in the language. midas (Metaphor Interpretation, Denotation, and Acquisition System) is a computer program that has been developed based upon this approach. midas can be used to represent knowledge about conventional metaphors, interpret metaphoric language by applying this knowledge, and dynamically learn new metaphors as they are encountered during normal processing. 2 1 Conventional Metaphor Consider the problem of understanding the conventional metaphoric language in the following examples. (1) How can I kill a process? (2) How can I get into Lisp? (3) You can enter Emacs by typing \emacs" to the shell. (4) Nili gave Marc her cold. (5) In ation is eating up our savings. The italicized words in each of these examples are being used to metaphorically refer to concepts that are quite distinct from those that might be considered the normal meanings of the words. Consider the use of enter in (3). Enter is being used, in this example, to refer to the actions on a computer system that result in the activation of a program. This use is clearly di erent from what might be called the ordinary or basic meaning of the word that has to do with the actions that result in an object entering an enclosure. While the word enter is used metaphorically in (3), this metaphor is neither novel nor poetic. Instead, the metaphorical use of enter results from a conventional systematic conceptual metaphor that allows computer processes to be viewed as enclosures. The various actions and states that have to do with the activation, deactivation, and use of computer processes are viewed as the standard actions and states that have to do with enclosures. This conceptual metaphor, structuring processes as enclosures, underlies the normal conventional way of speaking about these processes. Therefore, the uses of the words get into in (2) and enter in (3) are the ordinary conventional ways of expressing these concepts that nevertheless involve a systematic, productive metaphor. 2 The Metaphoric Knowledge Approach The main thrust of the Metaphoric Knowledge approach to metaphor is that the interpretation of metaphoric language proceeds through the direct application of speci c knowledge about the metaphors in the language. Moreover, it is asserted that the mechanisms that are used to apply this knowledge should be fundamentally the same as those used to interpret direct non-metaphorical language. Under this view, the proper way to approach the study of metaphor is to study the underlying details of individual metaphors and systems of metaphors in the language. This approach follows on the metaphor work of Lako and Johnson (Lako and Johnson, 1980) and the computational approaches to metaphor described in (Jacobs, 1985; Martin, 1986; Martin, 1987; Martin, 1988; Norvig, 1987). It is useful here to consider an analogy between the study of metaphor and the study of syntax. Broadly speaking, the study of syntax is concerned with the representation, use and acquisition of sets of complex facts that may be said to represent the grammar of a language. The approach to metaphor, described here, proceeds in a similar fashion. In 3 particular, it addresses the representation, use and acquisition of explicit knowledge about the metaphors in the language. This approach has been embodied in midas (Metaphor Interpretation, Denotation, and Acquisition System)(Martin, 1988). midas is a set of computer programs that can be used to perform the following tasks: explicitly represent knowledge about conventional metaphors, apply this knowledge to interpret metaphoric language, and learn new metaphors as they are encountered. Note that the term metaphor has historically been applied to a wide range of disparate phenomena. These have ranged from Kuhnian-shift type rethinkings of entire conceptual domains, to explicit formulaic simile statements like Man is a Wolf. It is important, therefore, to identify the particular kind of phenomena that the midas approach addresses. midas is solely concerned with modeling the everyday natural language task faced by readers of ordinary text. In particular, the quick and correct interpretation of ordinary text containing usually unnoticed commonplace metaphors. In order to make the problem of understanding metaphors more concrete, consider the implications of (1) through (3) for a system like the unix Consultant (Wilensky et al., 1984; Wilensky et al., 1988). uc is a natural language consultant system that provides naive computer users with advice on how to use the unix operating system. Metaphors like those shown above are ubiquitous in technical domains like unix. A system that is going to accept natural language input from users and provide appropriate natural language advice must be prepared to handle such metaphorical language. midas has been integrated into uc in order to give it the ability to handle this kind of metaphoric language. Perhaps more importantly, unix o ered an ideal domain, rich in metaphors, in which midas could be tested. Consider the following uc session illustrating the processing of a series of user queries. # How can I kill a process? Applying metaphor: Killing-Terminate You can kill a process by typing ^ C to the shell. # Tell me how to get out of emacs. Applying metaphor: Exit-Emacs You can get out of emacs by typing ^ X^ C. # Do you know how to enter lisp? Applying metaphor: Enter-Lisp You can enter lisp by typing ``lisp'' to the shell. In each of these examples, uc/midas attempts to nd the most coherent interpretation of the user's question, given its current knowledge of the conventions of the language. This involves checking the structure of the input against the constraints posed by all the possible conventional metaphorical and non-metaphorical interpretations. In each of these examples, 4 the only coherent interpretation is the one found through the application of a known unix metaphor. Consider the details of the Enter-Lisp metaphor in the last example. The knowledge that midas has of this metaphor speci es that the action of invoking certain kinds of processes can be viewed as an entering action, where the process invoked plays the role of the enclosure, and that the user performing the action is viewed as entering the enclosure. midas uses its knowledge of these conventional associations to infer that the use of enter by the user is most appropriately interpreted as a process invocation. 3 Previous Computational Approaches The Metaphoric Knowledge approach, embodied in midas, developed as a reaction to the strategies employed in previous computational approaches to metaphor. These approaches have adopted, what I call, a knowledge-de cient approach. By this, I mean an approach that makes no use of explicit knowledge about the metaphors in the language. These approaches do, of course, make use of other kinds of knowledge. They are de cient only with respect to explicit knowledge of the metaphorical conventions of the language. The knowledge-de cient approach has been manifested in two distinct processing strategies lying at opposite ends of a spectrum involving the representation and use of language conventions. The word-sense strategy (Hirst, 1987; Wilensky and Arens, 1980; Riesbeck, 1975; Small and Rieger, 1982) recognizes that there are conventional uses of words that deviate from ordinary compositional, or literal, meaning. This strategy addresses the problem by listing each separate use as an isolated and unmotivated word-sense in the lexicon. Under this approach the uses of enter lisp and get out of emacs in the previous examples are handled by distinct isolated lexical entries. While this approach adequately allows known conventional senses to be interpreted correctly, it nevertheless has a number of serious shortcomings. The listing of each separate use as an isolated fact in the lexicon simply fails to capture the generalizations among senses of di erent words or among the senses of a single word. Consider that a system that had knowledge about the use of get out of would not be able to handle related uses of exit or leave without listing them as separate facts. This failure to capture generalizations among word-senses provides the system with no basis for the prediction or classi cation of new uses as they are encountered. At the opposite end of the spectrum, lie approaches that are based on analogy or similarity (Carbonell, 1981; DeJong and Waltz, 1983; Fass, 1988; Gentner et al., 1988; Indurkhya, 1987; Wilks, 1978). These approaches assert that metaphors arise from an underlying conceptual similarity or analogy between the concepts representing the literal meaning of the words and the concepts underlying the ultimate meaning of the utterance. These approaches are at the opposite end of the spectrum because they use no knowledge about the conventions of the language. In particular, there is no use of any knowledge about metaphors that are a conventional part of the language. The task of interpreting metaphoric language is seen as a special purpose problem-solving task requiring access to knowledge and inference 5 techniques that are not otherwise a part of the normal language processing faculties. Note that the computational costs of these analogy mechanisms are radically higher than those posed for direct non-metaphorical language. While the details of each approach di er, they are all fundamentally based on the stage model (Searle, 1979) where the literal meaning of the sentence is computed and judged to be ill-formed, and then an analogy system is employed to search for an appropriate target meaning. The metaphoric knowledge approach taken in mida