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“Don’t judge a book by the cover” is an expression that is metaphoric and indicates that a person should not use the appearance of a thing to determine the value of its contents. The principle put forth by this idiom is conveyed through the idea of a book and its cover. Many people see a book cover and use the design or image upon it to decide whether the book itself is worth reading. When someone says “don’t judge a book by the cover,” he or she means that someone should look deeper or closer into something before deciding on its value.
The history of this popular proverb is somewhat difficult to determine, though it likely has an American origin. People have used the phrase “don’t judge a book by the cover” in numerous English-speaking countries, however, and it has been used in the US and the UK since the early to mid 20th Century. The idea expressed by this idiom, that of not judging someone or something at face value, is likely older than the actual phrase itself. Since the wording relies on books, specifically books with graphic or printed covers, however, it is a fairly recent expression that relies on a rather modern item.
As an expression, “don’t judge a book by the cover” means that someone should look deeper at something before he or she judges the merits of that thing. The basic idea is that people often look at the cover of a book and see the image the publisher has decided to place upon it. By looking at this image, people can then decide what the contents of the book are likely to be and choose whether or not to read it based on that image. There is no guarantee, however, that the cover of a book is truly representative of the contents of it.
This basic idea can then be metaphorically extended to people or objects in the world that are not books. Someone who may appear poorly dressed could potentially be very wealthy; he or she may simply choose not to wear clothes that indicate that wealth. The reality of human nature, however, is that many people do quickly pass judgment on objects and other people based on an initial appraisal of appearance and demeanor. Although “don’t judge a book by the cover” may be a fair policy when someone meets a new person, the ability to quickly evaluate a situation is a survival instinct that has remained with people and can potentially still serve to identify dangerous situations.