大般涅槃經集解 56.96

56.96

案僧亮曰：答第四難也。寶亮曰：明善心不退也。善心不生則已，生則無退。善若可退，云何成佛耶？但以遲得，故名退耳。

Sengliang states: This answers the fourth objection. Baoliang states: He explains that the wholesome mind does not retrogress. If the wholesome mind does not arise, that is all; once it has arisen, it does not retrogress. If good could retrogress, how could buddhahood be attained? It is merely because of late attainment that it is termed 'retrogression'.