水乳交融

Origin and story

Water and milk form an old comparison for close harmony in Buddhist writing. A biography of the Tang monk Shangheng in the third collection of eminent monks’ biographies, volume 16, describes his friendships after moving to Longxing Temple. Their accord is compared both to instruments sounding together and to water and milk assisting one another. The wording there is 水乳相資, a related expression rather than the exact modern idiom.

The full phrase 水乳交融 is securely attested in later fiction, including Lao Can Youji, chapter 19. It describes relationships or elements that combine so closely that separation feels unnatural. The earlier passage is a plausible part of the image’s development, not proof of one inventor or a single decisive event.