Metaphor. Tony Veale
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a highly conventional metaphor—establishes a frame of thought that encourages us to think in a particular way. Once it has our attention and draws us in, we are free to explore it, question it, and customize it as we see fit. For instance, we might ask how pulling on metaphorical strings might influence another person. If this other person is a weak-willed puppet, a lightweight player, or a minor cog in the machine (notice how effortlessly one metaphor leads to others), then not much effort is needed to exert influence, and so its strings will be light and easy to pull. But if this other person is a significant cog—a so-called big wheel or a heavyweight player—greater effort is needed to achieve any influence, requiring strings of greater thickness and tensile strength. Notice how the metaphor encourages us to think in the source domain (the domain of strings, cogs, pulleys, puppets, etc.) and to transfer our insights from here into the target domain (the domain of corporate decision-making). In Stone’s example, his metaphor leads us to believe that his friend Evan needed to perform Herculean efforts on his behalf, to influence some very powerful people at Google by pulling on some very heavy-duty strings. As we’ll see repeatedly throughout this book, even the most innocuous metaphors conceal a wealth of complexity, both in terms of their underlying representations and the cognitive/computational processes that are needed to understand them. This hidden complexity is a large part of what gives metaphors their allure for the computationalist.
However, although metaphor has a long and illustrious history of academic study, in both philosophy and linguistics, it remains a niche area in the computational study of language. For although most Natural Language Processing (NLP) researchers would readily acknowledge the ubiquity of metaphor in language, metaphor is a complex phenomenon and a hard engineering problem that continues, for the most part, to be wait-listed by the NLP community. To the application-minded, there are simply too many other problems of more practical and immediate interest—concerning syntax, semantics, inference, sentiment, co-reference, under-specification, and so on—that jump to the top of the community’s collective to-do list. That metaphor touches on all of these problems and more is often seen as beside the point, although ultimately it is very much to the point: the problem of metaphor is just too big, too unwieldy, and too knowledge-hungry to be tackled first. Better to get a handle on all the other problems first, to obtain an algorithmic understanding of the workings of language that can later be enriched by a computational model of metaphor. This makes good engineering sense, if little philosophical or cognitive sense.
The purpose of this book is to demonstrate that this conventional wisdom is predicated upon a false dichotomy: researchers can build figurative-language processing systems that are practical and efficient and cognitively plausible, and which also reflect an understanding of the profound philosophical issues involved. Indeed, it is difficult for the computationally-minded researcher to explore aspects of metaphor that have not been previously visited by philosophers or psychologists or by earlier computationalists in its long and illustrious history of academic analysis. If there is little in the field of metaphor that one can truly call “virgin territory,” it is nonetheless a field of many interesting landmarks that rewards careful viewing and repeat visits. We have written this book to be a comprehensive guide to the major landmarks in the computational treatment of metaphor and hope the reader will find it a useful map to this fascinating phenomenon’s many attractions.
CHAPTER 2
Computational Approaches to Metaphor: Theoretical Foundations
2.1 THE WHAT, WHY AND HOW OF METAPHOR
Metaphor is both pervasive and evasive: ubiquitous in language, yet remarkably hard to pin down in formal terms. Yet the fact that there exists no single, definitive perspective on metaphor is very much in keeping with its chimerical nature. For metaphor is a highly productive mechanism that allows us to create a panoply of viewpoints on any concept we care to consider, including metaphor itself. Indeed, it scarcely seems possible to say anything meaningful at all about metaphor without first resorting to one kind of metaphor or another. So we talk of tenors and vehicles, sources and targets, spaces and domains, mappings and projections, metaphors that are living or dead, those that are fresh but soon stale, or of seeing one idea with the aid of another. Many of our metaphors for metaphor are metaphors of seeing, for seeing often leads to knowing, or at least to an impression of knowing. So a metaphor can be a viewfinder, a magnifying glass, a microscope, a lens, a window, a pair of conceptual spectacles, a distorting fun-house mirror, and even a set of conceptual blinders. Max Black, the philosopher who more than anyone else kickstarted the modern resurgence in metaphor research, described a metaphor as working much like a blackened piece of glass onto which a specific pattern of lines has been etched [Black, 1962]. Looking through this bespoke glass, we see only those parts of a target domain that the etched pattern allows us to see, and see these parts in clearer relation to each other precisely because we are not distracted by the many parts we cannot see. Metaphor prompts us to look, helps us to see, and then controls what we can see.
Metaphor has existed longer than our need to describe its function, and much longer than our need to name it as a distinct linguistic or cognitive phenomenon. Indeed, the word “metaphor” is itself founded on a metaphor, to carry (-phor) above and across (meta-) a signifier from one realm of experience to another. The Greek philosopher Aristotle, to whom we can trace the earliest use of the term, effectively describes metaphor as a form of semiotic displacement, in which our agreed signifier for one object, idea, or experience is deliberately displaced onto another. But metaphor is not the only kind of semiotic displacement in language. Punning, for instance, involves the temporary replacement of one signifier with another, based on the phonetic similarity of the two, as in “mail” and “male” (e.g., “Q: Is that the mail plane coming in to land? A: Of course it’s a male plane, can’t you see its undercarriage?”). Synecdoche licenses the displacement of a signifier of a part onto the whole to which it belongs (e.g., “brains” stands for clever (or brainy) people in “the American and Russian militaries competed to round up the brains of the German rocket program”), while metonymy licenses a more general displacement of a signifier onto an object or idea that is functionally or conceptually related to its conventional signification (e.g., when Raymond Chandler’s private investigator Philip Marlowe says “Trouble is my business,” he means “My business concerns other people’s troubles, which invariably cause trouble for me;” so the metonymy tightens the relation between Marlowe and trouble). Aristotle is thus careful to enumerate the particular kinds of displacement that constitute a metaphor, as opposed to any other kind of semiotic play, and in his Poetics he offers the following schematic specification (as translated by Hutton [1982]):
Metaphor is the application to one thing of the name belonging to another. We may apply:
(a) the name of a genus to one of its species, or
(b) the name of one species to its genus, or
(c) the name of one species to another of the same genus, or
(d) the transfer may be based on a proportion.
The target of the name shift is typically called the topic, the tenor, or simply the target of the metaphor, while the source of the shift—the name belonging to another—is typically called the vehicle or simply the source of the metaphor. The designations source and target are generally preferred in the analogical literature (e.g., Gentner [1983]), although vehicle is more in keeping with Aristotle’s view of metaphor as a carrier of meaning across domains. Throughout this book we shall use the terminology of source domain and target domain to denote the space of ideas associated with the source (or vehicle) and the target (or tenor) respectively. The word domain has no formal definition; we use it, as others do, to designate the cluster of related concepts, properties, and norms that attach to a particular source or target so that we may later speak of transferring, mapping, or projecting content from one domain onto another. With this in mind, Aristotle offers an illustrative use-case for each of his four displacement strategies.
(a) Genus to species: “Here stands my ship” employs the generic (genus) term