This definition of the Daoist concept of xing and ming (nature and life) comes from the middle harmony anthology by Li Daochun. Li was a Yuan dynasty Daoist cultivator who formalized many ideas in the internal elixir meditation school and combined the earlier teachings of the southern and northern internal elixir schools of Zhang boduan and Wang chongyang.
夫性者，先天至神一靈之謂也。
Xing is the pre heaven utmost one spirit of the consciousness.
命者，先天至精一氣之謂也。
Ming is the pre heaven utmost one energy of the body’s essence.
精與性，命之根也。
When essence meets with xing, it is the root of life.
性之造化系乎心，命之造化系乎身。
Xing develops with the mind.
Ming develops with the body.
Comment: we can see here that xing and ming are absolutely connected to mind and body and that each of them exists in the latent, pre heaven state, which is the state under regular consciousness in which the body operates on its own, without our direct conscious interference.
Practicing xing and ming can only be done by returning the mind to absolute stillness and silence, and by setting the attention in such a way that it stays in one place and does not move. Xing and Ming are the big root of internal elixir meditation practice and should not be overlooked.