知人知面不知心 Know the face, not the heart
Explanation
这句俗语指认识一个人的外表容易，但了解其内心想法却很难。人不可貌相，海水不可斗量。
This proverb means that it's easy to know a person's appearance, but it's very difficult to understand their inner thoughts and intentions. One shouldn't judge a book by its cover.
Origin Story
从前，有个村子来了个算命先生，他相貌堂堂，衣着华丽，村民们都以为他是位德高望重的长者，纷纷向他请教。但实际上，这位算命先生却是个骗子，他用一些障眼法和花言巧语骗取了村民们的钱财，最后逃之夭夭。这件事让村民们明白了一个道理：知人知面不知心，不能只看外表，要用心去了解一个人。 另一个故事发生在战国时期。魏王要任命一位将军，有两个候选人，一个相貌威武，一个相貌平平。魏王倾向于选择相貌威武之人，但大臣张仪却建议魏王仔细考察二人的才能和品德，不能只看外表。魏王采纳了张仪的建议，最终选择了一位能力出众的将军，取得了战争的胜利。这个故事也说明，知人知面不知心，不能只看外表，而要注重内在素质。
Once upon a time, a seemingly wise old man arrived in a village. Dressed in fine robes, he seemed highly respected. The villagers, impressed by his appearance, eagerly sought his advice. However, this old man was a charlatan, using tricks and sweet words to deceive them. He ultimately left with their money. This incident taught the villagers the valuable lesson of "zhi ren zhi mian bu zhi xin." Appearances can be deceiving, one must discern the true nature of a person. In another instance, a king was selecting a general. He was torn between a candidate of formidable appearance and one of average looks. A wise advisor urged him to evaluate their abilities and character, not just their looks. The king heeded the advice, choosing the more capable candidate, leading to victory in war.
Usage
常用于形容人与人交往的复杂性，以及不能仅仅从外表判断一个人。
Often used to describe the complexity of human interactions and the fact that one cannot judge a person solely by their appearance.
Examples
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只知其表，不知其里；知人知面不知心。
zhī zhī qí biǎo, bù zhī qí lǐ; zhī rén zhī miàn bù zhī xīn
Only the surface is known, not the inside; one knows the face but not the heart.
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与人相处，要知人知面不知心。
yǔ rén xiāng chǔ, yào zhī rén zhī miàn bù zhī xīn
In dealing with people, one should know that one knows the face but not the heart.