Suppose you say something metaphorical, and by means of that you convey to me a content p. I now stipulate that "It's zinging" expresses precisely the content you conveyed. Technically, "It's zinging" is a zero-place predicate, like "It's raining." And now I say: "It's zinging." The literal content expressed by "It's zinging" is now equal to the metaphorical content conveyed by what you said. A third party can then pick up the phrase "It's zinging" from me without having heard the original metaphor, get a vague idea of its literal content from observing my use of it, and now a literal statement which has the same content as was conveyed by the metaphorical statement can start roaming the linguistic community.
Thus: If you cannot say something literally, you cannot whistle it either. For if you could convey it by whistling, you could stipulate a zero-place predicate to mean that which the whistling conveys.Objection 1: My grasp of "It's zinging" is parasitic on your metaphor, while the third-party doesn't have any understanding.
Response: Yes, and so what what? I wasn't arguing that you can usefully get rid of metaphor. It may well be essential to understanding the content in question. My point was simply that there can be a statement whose literal semantic content is the same as the content conveyed by your metaphor. Understanding is something further. This is very familiar in cases of semantic deference. (I hear physicists talking about a new property of particles. I don't really understand what they're saying, but I make the suggestion that they call that property: "Zinginess." My suggestion catches on. I can say: "There are zingy particles", and what I say has the same content as the scientists' attribution of that property. But while the scientists understand what they're saying, I have very little understanding.) The third party who hasn't heard the original metaphor may not understand much of what he's saying with "It's zinging." But what he's saying nonetheless has the literal semantic content it does by deference to my use of the sentence, and my use of the sentence has the literal semantic content it does by stipulation. All this is quite compatible with the claim that any decent understanding of "It's zinging" will require getting back to the metaphor. But, nonetheless, "It's zinging" literally means what the metaphor metaphorically conveyed.
Objection 2: The stipulation does not succeed. (This is due to Mike Rea.)
Response: Why not? If I can refer to an entity, I can stipulate a name for it, no matter how little I know about it. I may have no idea who killed certain people, but I can stipulate "Jack the Ripper" names that individual. My stipulation will succeed if and only if exactly one individual killed those people. Similarly, if I can refer to a property, I can stipulate a one-place predicate that expresses that property. (If a certain kind of Platonism is true, this just follows from the name case: I name the property "Bob", and then I have the predicate "instantiates Bob".) In cases without vagueness, contents seem to be propositions, and zero-place predicates express propositions, so just as I can stipulate a one-place predicate to express a property, I should be able to stipulate a zero-place predicate to express a propositions. And in cases of vagueness, where maybe a set of propositions (or, better, a weighted set of propositions) is a content, I should be able to stipulate a similarly vague literal zero-place predicate as having as its content the same set of propositions.
There are many ways of introducing a new term into our language. One way is by stipulating it in terms of literal language. That's common in mathematics and the sciences, but rare in other cases. Another way is by ostension. Another is just by talking-around, hoping you'll get it. One way of doing this talking-around is to engage in metaphor: "I think we need a new word in English, 'shmet'. You know that butterflies in the stomach feeling? That's what I mean." We all understand what's going on when people do this kind of stipulation. For all we know, significant parts of our language came about this way.