The dust-jacket was a late 19th-century invention; the notion that 'you can't judge a book by its cover' must be a good deal older than that. It's an expression that in an ideal world no one would ever say. But people do say it - often, oddly, when they're talking about books. I say 'oddly' because once upon a time a book's cover would have given little indication of its contents and done less to distinguish it from the other volumes beside it on the shelf. So the thrust of the cliché - if clichés can be said to have thrust; perhaps the definition of a cliché is a phrase that has lost its thrust - would have been that, just as you can't judge a book by its cover, which is a self-evident fact, so you shouldn't base your opinion of anything else on surfaces or first impressions: you can't judge an accused man by his physiognomy. Fine. But as the number of books jostling for readers' attention has grown, publishers have toiled to make it not only possible but ever easier to judge books by their covers. So to say that 'you can't judge a book by its cover' and mean it literally is to use an assertion that is neither self-evident nor entirely true as an analogy for itself: I think this is what's known as begging the question.
LRB 20 July 2006 | PDF Download