Laozi 21, in Wing-Tsit Chan, Chinese Philosophy, Chapter 7.
Philosophically this is the most important chapter of the book. The sentence "The essence is very real" virtually formed the backbone of Chou Tun-i's (Chou Lien-hsi, 1017-1073) Explanation of the Diagram of the Great Ultimate, which centers on the "reality of the Non-Ultimate and the essence of yin and yang." And Chou's work laid the foundation of the entire Neo-Confucian metaphysics. Of course Neo-Confucian metaphysics is more directly derived from the Book of Changes, but the concepts of reality in the Book of Changes and in this chapter are surprisingly similar.