What does St. Augustine say about chastity of heart?

St. Augustine says, “Lest continence only from the lust of the inferior parts of the flesh seem to be hoped for from God as necessary, in the psalm there also is sung this: ‘Set a watch, O Lord, before my mouth and a door of continence round about my lips’ (Ps. 141:4). Now, in this testimony of divine eloquence if we understand ‘mouth’ as we ought to understand it, the watch placed there is continence, inasmuch as we understand it as a gift of God. Surely, it is a slight matter to restrain the mouth of the body lest something that is not expedient come forth from it through the sound of the voice. Within is the mouth of the heart where he who said those words and directed us to say them desired that a guard and gate of continence be set for him by God.”⁸¹¹