What term is best associated with the phrase "don't judge a book by its cover"?

The phrase "don't judge a book by its cover" is best associated with the term idiomatic expression. An idiomatic expression is a phrase or an expression whose meaning cannot be understood from the ordinary meanings of the words that make it up. This specific phrase conveys the idea that outward appearances can be misleading and encourages looking beyond the surface to understand the true nature of something or someone. It reflects a common sentiment in many cultures and serves as a reminder about the importance of deeper understanding and not forming quick judgments based solely on superficial characteristics.

While the other terms are relevant in different contexts, they do not encapsulate the essence of the phrase as effectively as "idiomatic expression." Common sense implies practicality and sound judgment, which, while related, does not capture the linguistic and cultural nuance of the phrase. A paradox refers to a statement that contradicts itself yet may reveal a truth, which is not applicable here. A rhetorical device involves techniques used in speaking or writing to persuade or impact an audience, but this phrase primarily functions as an idiomatic expression reinforcing a moral lesson.