bù èr fǎmén
Literal meaning: “[The] Dharma Door of Non-duality”
Idiomatic meaning: “the one and only way”
The original Buddhist meaning is very different from the common vernacular meaning today. “Non-duality” refers to the Middle Path of Buddhism, which threads between two extremes. This particular chengyu is derived from the Vimalakīrti Nirdeśa Sūtra, and the following passage illustrates the non-duality of existence and non-existence.