 They are generated in the first place by the division of original Unity into Yin and Yang, and by the further subdivision of Yin and Yang into four states.In the "Cantong qi", Water and Fire are the Yin and Yang of the postcelestial state, and Wood and Metal are True Yin and True Yang of the precelestial state.Soil, the fifth agent, has both a Yang and a Yin aspect.