People churn milk because they understand that butter is present therein. Why do people not churn water? Because that substance is not present there. Likewise, people maintain moral discipline and engage in the holy life because of the existence of Buddha Nature.
Moreover, people who want gold and are endowed with discernment, dig in earth. Why do they not dig in trees? They dig in rocks where gold-ore is present, but they do not dig in trees, where there is no gold. Likewise, people who discern the presence of the Buddha Nature think to themselves, “I shall become a Buddha,” and so maintain the moral discipline and engage in the holy life.
Furthermore, if there were no Buddha Nature, the holy life would be pointless. Just as butter will never be produced from water even if one were to churn it for a billion years, similarly there would be no benefit for those who engage in the holy life and the moral discipline if there were no Buddha Nature.
—Venerable Angulimala, Angulimaliya Sutra