Chuang Tzu, ch. 7, NHcc, 3:35b-36a, in Wing-Tsit Chan, Chinese Philosophy, Chapter 8.
The mirror is an important symbol for the mind both in Zen Buddhism and in Neo-Confucianism. The difference is that with Buddhism, external reality is to be transcended, whereas with Chuang Tzu and Neo-Confucianists, external reality is to be responded to naturally and faithfully, like a mirror objectively reflecting all. (see Paul Dernieville, "Le miroir spirituel," Sinologica 1 (1948), especially pp 117-119).