I recently came across this phrase in its modern meaning, which is that one should not let outward appearances that suggest the interior is bad deceive one into thinking that the interior is bad without looking. Sometimes it is used to mean that a humble exterior should not mislead one into thinking that the interior is not great; sometimes it means that a morally bad exterior shouldn’t mislead one into thinking that the interior is rotten. Curiously, these are all opposite to the original meaning, which is that one should not be mislead by an attractive cover into assuming that the interior is as good.
At least according to Wikipedia (which, admittedly, can only be trusted about as far as one can throw the entire server cluster its runs on in a single throw), the original of not being able to judge a book by its cover was from The Mill On the Floss by George Eliot.
‘The History of the Devil,’ by Daniel Defoe,—not quite the right book for a little girl,” said Mr Riley. “How came it among your books, Mr Tulliver?”
Maggie looked hurt and discouraged, while her father said,—
“Why, it’s one o’ the books I bought at Partridge’s sale. They was all bound alike,—it’s a good binding, you see,—and I thought they’d be all good books. There’s Jeremy Taylor’s ‘Holy Living and Dying’ among ’em. I read in it often of a Sunday” (Mr Tulliver felt somehow a familiarity with that great writer, because his name was Jeremy); “and there’s a lot more of ’em,—sermons mostly, I think,—but they’ve all got the same covers, and I thought they were all o’ one sample, as you may say. But it seems one mustn’t judge by th’ outside. This is a puzzlin’ world.”
I’ve never read George Eliot, and the little bit I’ve read here does not make me wish to remedy that. There is, however, a cute little bit that comes not long after the above:
“Here he is,” she said, running back to Mr Riley, “and Tom coloured him for me with his paints when he was at home last holidays,—the body all black, you know, and the eyes red, like fire, because he’s all fire inside, and it shines out at his eyes.”
“Go, go!” said Mr Tulliver, peremptorily, beginning to feel rather uncomfortable at these free remarks on the personal appearance of a being powerful enough to create lawyers; “shut up the book, and let’s hear no more o’ such talk. It is as I thought—the child ’ull learn more mischief nor good wi’ the books. Go, go and see after your mother.”
Mr. Tulliver is not, from what I gather, meant to be a learned man, nor an intelligent man, and moreover I presume given the time period (1830s) he was an English protestant. Still, to assign the power to create to the devil is… especially bad theology. It’s an amusing turn of phrase, though.
Incidentally, this also reminds me of a passage from G.K. Chesterton’s What’s Wrong With the World:
I am by no means sure that even in point of practical fact that elegant female [of Victorian times] would not have been more than a match for most of the inelegant females [of modern times]. I fancy Jane Austen was stronger, sharper and shrewder than Charlotte Bronte; I am quite certain she was stronger, sharper and shrewder than George Eliot. She could do one thing neither of them could do: she could coolly and sensibly describe a man.
From what very little I’ve seen of them, George Eliot’s male characters certainly seem to be caricatures.
Be that as it may, it is very curious that we’ve taken an idea which primarily referred to not being seduced by promising externalities and turned it into advice to avoid being cautious based on obvious warning signs. I can’t help but wonder whether it’s because these days vicious people so rarely bother even with the pretense of virtue, and they think it rude of people to notice.
(That said, I must of course note that there can be good books with covers that aren’t nearly as good. One can certainly find these among the many self-published books of our day. If it comes to that, many of the mystery books published by big publishers have very cheap covers that don’t look like much of anything. On one copy I have of the complete Father Brown mysteries, if you went by the cover you would assume the book contains a five year old’s scribblings, if, granted, a precocious five year old.)