The Chinese idiom 老生常谈 (lao3 sheng1 chang2 tan2) literally means the mere platitudes or commonplace talk of an old scholar. The idiom comes from a story about an "old scholar" called Guan Lu who lived in the State of Wei during the Three Kingdoms period (AD 220-280).
Guan was a child prodigy and by the tender age of 15 he was renown for his scholarly abilities. He was so well versed in classical literature and astronomy that he was invited to a local magistrate's home to discuss these matters with more than 100 philosophers. These philosophers tried to completely stump the young prodigy with curly questions, but it was to no avail.
Guan's prestige continued to grow and officials sought him out for advice. One day, two ministers of the imperial court asked Guan to fortell their fortunes and career paths because they had both had bad dreams the previous night. Guan learned that the two officials were nasty pieces of work. They were vindictive and greedy and despised by their colleagues and subordinates. So Guan told the officials that their dreams indicated a gloomy future beset with many problems and difficulties.
One minister was like a cat on a hot tin roof - he became very anxious and worried about what Guan had told him. The other minister tried to console him by telling him that Guan's words were the commonplace talk of a scholar and that he should not work himself into a lather over nothing.
However, several months later both ministers were executed for their involvement in a bungled coup. Guan subsequently told his friends that this was a typical example of what can happen if people ignore the truth expressed by a commonplace notion. English speakers call these commonplace notions Sunday school truths.