(1) God is my copilot.
Now suppose that someone asked you to explain precisely what this sentence meant. You might be able to say various things that would provide a sense of more or less what the sentence was supposed to convey, but it seems that you would never be able to capture the full richness of the metaphor itself: inevitably, you would end up leaving something out. A number of philosophers have pointed to this striking difficulty we face in explaining the meaning of metaphors and used it as the basis for philosophical theories about the nature of metaphor itself.
In a forthcoming paper, Mark Phelan takes up this supposed fact about our inability to paraphrase metaphors and subjects it to actual empirical study. But Phelan introduces a surprising new twist. Instead of simply asking people to explain what metaphors mean, he compares people's ability to explain the meanings of metaphors with their ability to explain the meanings of perfectly literal sentences. So, for example, people's ability to explain the meaning of sentence (1) could be compared with their ability to explain the meaning of the sentence:
(2) Bill Thompson is my copilot.
Phelan asked experimental subjects to give paraphrases for metaphorical sentences; then he took these paraphrases and presented them to other experimental subjects, asking them whether the paraphrase captured the full meaning of the original or whether it left something out. Just as one might predict, subjects felt that the paraphrases inevitably left something out. This confirms the usual philosophical view. However, when Phelan did the same with paraphrases of perfectly literal sentences, he got exactly the same result! In fact, he sometimes found that subjects were, if anything, more likely to think that something had been left out of the paraphrases of literal sentences than they were to think that something had been left out of the paraphrases of metaphorical sentences.
In short, it looks like it really is pretty impossible to explain what a metaphor means. But that is not because of anything special about metaphors. It is merely a reflection of the fact that we can't explain what any sentence means.