2 And lest it should seem that necessary Continence was to be hoped for from the Lord only in respect of the lust of the lower parts of the flesh, it is also sung in the Psalm; “Set, O Lord, a watch to my mouth, and a door of Continence around my lips.” But in this witness of the divine speech, if we understand “mouth” as we ought to understand it, we perceive how great a gift of God Continence there set is. Forsooth it is little to contain the mouth of the body, lest any thing burst forth thence, which is not for the better, through the sound of the voice.
For there is, within, the mouth of the heart, where he, who spoke these words, and wrote them for us to speak, desired of the Lord that the watch and door of Continence should be set for him. For many things we say not with the mouth of the body, and cry aloud with the heart: but there goes forth from the mouth of the body no word of any thing, whereof there is silence in the heart. Therefore what flows not forth thence, sounds not abroad: but what flows forth thence, if it be evil, although it move not the tongue, defiles the soul.
Therefore Continence must be set there, where the conscience even of them who are silent speaks. For it is brought to pass by means of the door of Continence, that there go not forth thence that, which, even when the lips of the flesh are closed, pollutes the life of him that has the thought.
Source: On Continence (New Advent)