This is a variant of the better known proverb “don’t judge a book by its cover”, and it carries the same warning: what something looks like on the outside tells you nothing reliable about what it is really like underneath. It is said of people whose manner, dress or background suggests one thing while their character or abilities turn out to be quite different, and of things, plans or situations that should be judged on substance rather than on how they present themselves. The phrase works because the comparison is so ordinary: a plain cover can wrap a brilliant book, and a glossy one can wrap a dull one, so the cover is no guide at all.
What’s the origin of the phrase ‘you can’t tell a book by looking at its cover’?
The idiom and its variants are generally treated as an American English coinage, used to mean making assumptions about someone or something from superficial appearance rather than genuine substance. The underlying metaphor, comparing judgement by outward show to judging a book, is much older than any of the fixed modern wordings. It turns up in Edmund Arwaker’s 1708 preface to Truth in Fiction, where the comparison is already being drawn. A similar idea appears in American political reporting in 1837, when a New York senator, Mr Roosevelt, was quoted arguing that it would be as ridiculous to presume fraud from the face of a petition “as to undertake to judge of a book from its title.”
The phrase is often loosely traced to George Eliot’s 1860 novel The Mill on the Floss, but that link does not really hold up: the passage usually cited refers to an actual physical book, not to a general saying about judging character by appearance, so the connection is at best a borrowed resemblance rather than a true source.
A wording close to the one used today, telling the reader they can never tell a book by its cover, is documented in the 1946 murder mystery Murder in the Glass Room, written by Edwin Rolfe and Lester Fuller. By the mid twentieth century the saying had settled into something close to its modern shape, with “tell” and “judge” used interchangeably and “looking at” sometimes inserted, as in this version, without changing the sense at all.