3 Lastly, to show more plainly the inner mouth, which by these words he meant, after having said, “Set a watch, O Lord, to my mouth, and a door of Continence around my lips,” he added straightway, “Cause not my heart to fall aside into evil words.” The falling aside of the heart, what is it but the consent? For he has not yet spoken, whosoever in his heart has with no falling aside of the heart consented unto suggestions that meet him of each several thing that is seen.
But, if he has consented, he has already spoken in his heart, although he has not uttered sound by the mouth; although he has not done with hand or any part whatever of the body, yet has he done what in his thought he has already determined that he is to do: guilty by the divine laws, although hidden to human senses; the word having been spoken in the heart, no deed having been committed through the body. But in no case would he have moved the limb without, in a deed, the beginning of which deed had not gone before within in word.
For it is no lie that is written, that “the beginning of every work is a word.” Forsooth men do many things with mouth closed, tongue quiet, voice bridled; but yet they do nothing by work of the body, which they have not before spoken in the heart. And through this since there are many sins in inward sayings which are not in outward deeds, whereas there are none in outward deeds, which do not go before in inward sayings, there will be purity of innocence from both, if the door of Continence be set around the inward lips.
Source: On Continence (New Advent)