天长地久 (tiān cháng dì jiǔ)

How the meaning moved

The four characters are the whole first line of chapter 7: “The cosmos is infinite and timeless.” Literally they say that heaven is long and earth is lasting. The chapter then gives the reason heaven and earth can last: they “do not foster themselves. Thus they can live long.” The sage learns from them: “The sage puts himself last, and thus is first”, and the chapter ends: “Is he not selfless? This is precisely the cause of his fulfillment.”

In the chapter the line is a statement about heaven and earth, and the point is the reason they last: they do not live for themselves. Later the phrase was lifted out of the chapter and used of time that never ends. The Han poet Zhang Heng wrote “heaven is long, earth is lasting, the years do not stay” in his “Rhapsody on Contemplating the Mystery” (思玄赋), and Bai Juyi closed his “Song of Everlasting Sorrow” (长恨歌) with “Heaven is long and earth is lasting, yet they will end one day; this sorrow goes on and has no end.” Today 天长地久 is said mostly of love and friendship, in vows and wedding wishes, often reversed as 地久天长.