He quoted 'uprightness lies within' to argue family members shouldn't denounce each other.
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From 《论语·子路》. A man of She boasted 'our upright ones testify when their father steals.' Confucius replied: 父为子隐,子为父隐,直在其中矣 — 'the father conceals for the son, the son conceals for the father — uprightness lies within that.' A foundational text in Chinese debates on law vs. family loyalty.
usage
矣 (yǐ) is a classical final particle of assertion, like modern 了. Only meaningful when quoted with the preceding 父为子隐,子为父隐. Still heavily cited in Chinese legal philosophy.