Per 《
新华成语词典》, derived from 《
吕氏春秋·
博志》 (3rd century BC):
全则必缺,
极则必反 — 'what is whole must become incomplete; what reaches its extreme must reverse.' Echoed in 《
鹖冠子·
环流》:
物极则反,
命曰环流 ('things at their extreme reverse — call this the circular flow'). Captures the Daoist intuition behind
阴阳 — every yang peak conceals an emerging yin.