For one person to try to stop the entire reform is no different from a mantis trying to halt a chariot.
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From 《庄子·人间世》: 怒其臂以当车辙,不知其不胜任也 - 'angrily raising its forelegs to block the chariot's track, unaware that it cannot prevail.' Zhuangzi's image for self-destructive overconfidence.
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Picture it literally: a mantis (螳) raising its arms (臂) to block (当) a chariot (车). The image is the meaning.