Relevance is a word that makes professors of economics wince these days. There was a time when I could initiate a class into the mysteries of diminishing marginal utility, explaining why the man in the Sahara Desert would not be willing to pay as much for the third pint of water as for the second, confident that when the hands went up it would be because someone was convinced that he ought to pay more, because his total utility was greater after three pints than after two. Today when the hands go up, I know what's coming: "That's clear enough, but I don't see how it's relevant."