Dao is energy of the void (unseen & ever-changing), therefore ever-lasting (changing from one stage to next). Dao is the womb, where changes are possible (birth, death, re-birth…death, birth…). All things that can be name (described), will eventually change with time & become no more…only Dao is ever changing, thus everlasting.
谷神 : Void Energy
玄牝 : The Womb