 Put differently, the Dao can only be described as wu because it does not have any distinguishable feature or property characteristic of things. On this reading, wu does not signify ontological absence, but on the contrary attests to the fullness and fecundity of the Dao. More precisely, through a process of differentiation, the Dao generates the yin and yang qi that constitute all phenomena. The Laozi has also made the point that the Dao is “undifferentiated and complete” (Chapter 25). This is now shown to be the source of the yin and yang qi—vital forces, pneumas, or loosely, “energies”—that engender, shape and sustain life. In this respect, He Yan adhered generally to the yin-yang cosmological theory established since the Han dynasty. Viewed in this light, the nothingness of Dao has important implications for ethics and political philosophy.