4 For which cause our Lord Himself also with His own mouth says, “Cleanse what are within, and what are without will be clean.” And, also, in another place, when He was refuting the foolish speeches of the Jews, in that they spoke evil against His disciples, eating with unwashen hands; “Not what enters into the mouth,” said He, “defiles the man: but what comes forth out of the mouth, that defiles the man.” Which sentence, if the whole of it be taken of the mouth of the body, is absurd.
For neither does vomit defile him, whom food defiles not. Forsooth food enters into the mouth, vomit proceeds forth out of the mouth. But without doubt the former words relate to the mouth of the flesh, where He says, “Not what enters into the mouth defiles the man,” but the latter words to the mouth of the heart, where He says, “But what proceeds forth out of the mouth, this defiles the man.” Lastly, when the Apostle Peter sought of Him an explanation of this as of a parable, He answered, “Are ye also yet without understanding?
Understand ye not, that whatsoever enters into the mouth, goes into the belly, and is cast out into the draught?” Here surely we perceive the mouth of the flesh, into which the food enters. But in what He next adds, in order that we might recognize the mouth of the heart, the slowness of our heart would not follow, did not the Truth deign to walk even with the slow. For He says, “But what things go forth from the mouth, go out of the heart;” as though He should say, When you hear it said “from the mouth,” understand “from the heart.”
I say both, but I set forth one by the other. The inner man has an inner mouth, and this the inner ear discerns: what things go forth from this mouth, go out of the heart, and they defile the man. Then having left the term mouth, which may be understood also of the body, He shows more openly what He is saying. “For from the heart go out,” says He, “evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies; these are what defile the man.” There is surely no one of those evils, which can be committed also by the members of the body, but that the evil thoughts go before and defile the man, although something hinder the sinful and wicked deeds of the body from following.
For if, because power is not given, the hand is free from the murder of a man, is the heart of the murderer forsooth therefore clean from sin? Or if she be chaste, whom one unchaste wishes to commit adultery with, has he on that account failed to commit adultery with her in his heart? Or if the harlot be not found in the brothel, does he, who seeks her, on that account fail to commit fornication in his heart? Or if time and place be wanting to one who wishes to hurt his neighbor by a lie, has he on that account failed already to speak false witness with his inner mouth?
Or if any one fearing men, dare not utter aloud blasphemy with tongue of flesh, is he on this account guiltless of this crime, who says in his heart, “There is no God.” Thus all the other evil deeds of men, which no motion of the body performs, of which no sense of the body is conscious, have their own secret criminals, who are also polluted by consent alone in thought, that is, by evil words of the inner mouth. Into which he (the Psalmist) fearing lest his heart should fall aside, asks of the Lord that the door of Continence be set around the lips of this mouth, to contain the heart, that it fall not aside into evil words: but contain it, by not suffering thought to proceed to consent: for thus, according to the precept of the Apostle, sin reigns not in our mortal body, nor do we yield our members as weapons of unrighteousness unto sin. From fulfilling which precept they are surely far removed, who on this account turn not their members to sin, because no power is allowed them; and if this be present, straightway by the motions of their members, as of weapons, they show, who reigns in them within.
Wherefore so far as is in themselves, they yield their members weapons of unrighteousness unto sin; because this is what they wish, which for this reason they yield not, because they are not able.
Source: On Continence (New Advent)