I've cautioned students against using metaphors; though they make writing more vivid, and sometimes more persuasive, they often obscure more than they reveal. Part of the problem is that they are literally false -- if they were true, they wouldn't be metaphors. Sometimes the literal falsehood reveals a deeper truth, but sometimes it's just plain false.

A case in point, from the debate over organ markets: I've often run into the argument that "We don't let prostitutes sell their bodies, so we shouldn't let people sell body parts." (For a print example, see Margaret Engel, Va. Doctor Plans Company to Arrange Sale of Human Kidneys, Wash. Post, Sept. 19, 1983, A9, quoting then-Representative Al Gore.) Sounds logical, no?

Except that "selling your body" is a metaphor. Prostitution doesn't actually involve sale of the body as a good. Rather, it involves the sale of services that use the body.

No problem, some might say; that's obviously implied by the phrase. But if we replace the literally false phrase with the literally accurate one -- "We don't let prostitutes sell services that use their bodies, so we shouldn't let people sell body parts" -- we notice something wrong with the argument: Prostitution is actually the exception rather than the rule when it comes to selling services using one's body. We often let people sell services that use their bodies; consider, among many others, people who work as furniture movers, nonsexual masseurs, non-sexual models, or professional athletes.

Of course, now that we're replacing false descriptions with accurate ones, we can clarify things further in a way that properly describes what's going on with bans on prostitution: "We don't let prostitutes sell sexual services, so we shouldn't let people sell body parts."

But once we've clarified things, we see that the analogy is entirely inapt: The problem with prostitution has little to do with commercialization of the body as such, and everything to do with commercialization of a specific kind of bodily services (sexual services). This is why being paid to use one's hands to massage someone's back is legal even though being paid to use the same hands to massage someone's genitals is a crime. And commercialization of sexual services has nothing at all to do with organ transplants.

Now of course this doesn't rebut the various other arguments against compensation for organs (though I've tried to rebut them in other posts, linked to below). But it does, I think, rebut this argument. And it illustrates the importance of (1) looking beyond the metaphors to the reality that they supposedly portray, and (2) using metaphors only when one has assured oneself that the metaphor (despite its literal falsehood) sufficiently matches the reality.