Species to genus: “Truly ten thousand noble deeds hath Odysseus done” uses “ten thousand deeds” as a specialization of the vague genus term “large number.”
(c) Species to species: “Drawing off the life with bronze” and “Cutting off the water with unwearied bronze” each employ a specialization of the genus “to take away.”
(d) Proportional analogy: “The wine cup is to Dionysus as the shield is to Ares” is an obvious statement of the proportion Dionysus:Cup::Ares:Shield, yet it supports more subtle allusions, such as the poetic use of “the cup of Ares” to refer to a battle shield, or “the shield of Dionysus” to refer to a wine cup.
So, metaphor distinguishes itself from other forms of semiotic displacement by its reliance on the vertical operations of generalization and specialization. Metaphor allows us to move signifiers up and down a hierarchy of concepts, and supports the sideways movement of signifiers only insofar as they achieve such movement by climbing up from one domain so as to climb down into another. In effect, Aristotle provides a strong account of the “what” of metaphor, and a reasonable account of the “how,” but offers very little on the “why” of metaphor, other than to suggest that it is done in a spirit of semiotic sport. The strategies in (a) to (d) above focus mainly on those uses of metaphor where the vehicle is explicitly given (e.g., the cup of Ares or ten thousand deeds) and a listener must work out the identity of the implied tenor (e.g., a shield, or a large number). But what of metaphors where tenor/target and vehicle/source are both explicitly given, as in the copula metaphor “marriage is slavery?”
Such metaphors appear to instantiate strategy (c) in Aristotle’s scheme, in which one species of an unstated genus is mapped to another of the same genus. Presumably it is the identification of this unstated genus term—perhaps cruel practice or social institution in the case of marriage as slavery—that offers the key to the metaphor. Yet, if so, the metaphor degenerates into an example of strategy (b) and communicates only as much information as this genus can offer. In fact, if the listener does not possess the same conceptual hierarchy as the speaker—for instance, the listener may not view even slavery as a cruel practice, much less marriage—then the metaphor will fail. However, one need not agree with a metaphor to understand its purpose, and many metaphors are designed to change a listener’s conceptual structures rather than to merely reflect them. So, in an important sense, the “why” of metaphor is precisely this: speakers often use metaphors to align the conceptual systems of others to that of their own.
For a metaphor is not a statement of fact, and most are more than the mere expression of a subjective viewpoint. Rather, a metaphor is an invitation to build a joint meaning space, as framed by the given viewpoint. The metaphor “marriage is slavery” is a communicative gambit that asks the listener to think about, and perhaps argue about, the idea that our concept of marriage can be meaningfully framed by our concept of slavery. In other words, the metaphor aims to establish what Brennan and Clark [1996] call a conceptual pact, and is a request that can be paraphrased as follows: “let us agree to talk about marriage using the words and ideas we normally associate with slavery; to this end, let us go so far as to suspend disbelief and assume that marriage really is a kind of slavery, and thereby explore the extent to which our concept of marriage can be subsumed under our concept of slavery.” The longwindedness of this paraphrase goes some way toward conveying the true utility of metaphor as much more than a flourish of language or a playful exercise in semiotic sport.
The metaphor “marriage is slavery” is an effective prompt to build a complex meaning space because “slavery” is itself an effective shorthand for a rich body of cultural knowledge and expectations. Wrapped up in our concept of slavery is a knowledge of the history of the practice, of the physical and emotional suffering of those who are caught in it, and of the motivations of those who practice it. In other words, slavery is a dense descriptor, an idea that is easily referenced and that brings with it a slew of properties, feelings, norms, and expectations. The use of a dense descriptor in a metaphor prompts a listener to unpack those aspects of the descriptor that seem relevant to the target/tenor. This conciseness comes at a price, as the listener may associate different feelings and properties with a concept like slavery, or may unpack the descriptor to focus on different aspects than those of interest to the speaker. For instance, a speaker may employ the concept of slavery to focus on the domestic abuse, emotional and physical, suffered by one who is trapped in a bad marriage, while the listener may instead unpack the reference to focus on the institutionalized nature of marriage and slavery. Yet the indeterminate nature of metaphor, a natural consequence of its concision, is often a desirable quality of a conceptual pact. One rarely uses metaphor to communicate a rigid set of facts or beliefs. Rather, metaphors introduce a set of talking points for a target, so that speaker and listener might converge toward a mutual understanding, if not of a target idea then of each other’s views of the target. The indeterminacy of metaphor makes it a flexible form of communication. While literal language commits a speaker to a fixed interpretation, and offers little scope for the listener to contribute to the joint construction of meaning, metaphorical language suggests a looser but potentially richer meaning that is amenable to collaborative elaboration by each participant in a conversation.
The densest descriptors are the familiar stereotypes that get used time and again in our metaphors and our similes. Although every concept has the potential to be used figuratively, casual metaphors tend to draw their dense descriptors from the large pool of stereotypes shared by most speakers of a language (see Taylor [1954]). Because so many familiar stereotypes have polarizing qualities—think of the endearing and not-so-endearing qualities of babies, for instance—they serve as ideal vehicles for a metaphor that aims to convey a strong affective stance toward a topic. Even when stereotypes are not used figuratively, as in the assertion “Steve Jobs was a great leader,” they are often likely to elicit metaphors in response, such as “yes, a pioneer,” or “a true artist!” or even “but what a tyrant!” Familiar proper-named entities can also be used as dense descriptors, as when Steve Jobs is compared to the fictional inventor Tony Stark, or Apple is compared to Scientology, or Google to Microsoft. Metaphors are flexible conceits that draw on dense descriptors to concisely suggest a position on a target idea while seeking elaboration or refutation of this position from others. Our computational models for the interpretation of metaphors must thus allow speakers to exploit the same flexibility of expression when interacting with machines as they enjoy with other humans. Such a goal clearly requires a computational system to possess, or acquire, a great deal of knowledge, usefully clustered into dense pockets of properties, feelings, and expectations that can be evoked with a single word or idea.
2.2 THE “MEANING” OF METAPHOR
If metaphor is as much a process as a product, what then is the meaning of any given metaphor? We can point to the linguistic rendering of a metaphor and say that this string is a linguistic metaphor. We can point to the conceptual structure underpinning this linguistic rendering—and to other renderings of the same conceit—and say that this deep structure is a conceptual metaphor. But to what can we point and say: this is the meaning of this metaphor? The problem of metaphor meaning can be approached from a number of perspectives. We can talk of the truth value of a metaphor, much as we can talk of the truth value of any proposition. We can talk of a specific interpretation of a metaphor. Or, we can talk of the specific inferences that a metaphor licenses in the mind of the hearer, as well as the emotions for the target that it is likely to engender in a hearer.
Although most metaphors are literally false, truth value is a remarkably poor guide to the metaphorical nature of any statement. The assertion that “Barack Obama is a Muslim” is no more a metaphor because it is false than “Al Gore is not a robot” is not a metaphor because it is true. For logicians and philosophers, meaning resides not in truth values but in truth conditions, the set of criteria that, if true, would make a statement true as a whole. For instance, in semantic theory in which “Barack Obama” denotes the President of the United States in 2015, and in which “Muslim” denotes the set of all people