St. Ambrose explains that the flesh given to Satan for destruction is eaten by the serpent when the soul is set free from carnal desires. He gives, therefore, various rules for guarding the senses, points out the snares laid for us by means of pleasures, and exhorts his hearers not to fear the destruction of the flesh by the serpent.
68. The serpent eats this dust, if the Lord Jesus is favourable to us, that our spirit may not sympathize with the weakness of the flesh, nor be set on fire by the vapours of the flesh and the heat of our members. “It is better to marry than to burn,” for there is a flame which burns within. Let us not then suffer this fire to approach the bosom of our minds and the depths of our hearts, lest we burn up the covering of our inmost hearts, and lest the devouring fire of lust consume this outward garment of the soul and its fleshy veil, but let us pass through the fire. And should any one fall into the fire of love let him leap over it and pass forth; let him not bind to himself adulterous lust with the bands of thoughts, let him not tie knots around himself by the fastenings of continual reflection, let him not too often turn his attention to the form of a harlot, and let not a maiden lift her eyes to the countenance of a youth. And if by chance she has looked and is caught, how much more will she be entangled if she gazes with curiosity.
69. Let custom itself teach us. A woman covers her face with a veil for this reason, that in public her modesty may be safe. That her face may not easily meet the gaze of a youth, let her be covered with the nuptial veil, so that not even in chance meetings she might be exposed to the wounding of another or of herself, though the wound of either were indeed hers. But if she cover her head with a veil that she may not accidentally see or be seen (for when the head is veiled the face is hidden), how much more ought she to cover herself with the veil of modesty, so as even in public to have her own secret place.
70. But granted that the eye has fallen upon another, at least let not the inward affection follow. For to have seen is no sin, but one must be careful that it be not the source of sin. The bodily eye sees, but let the eye of the heart be closed; let modesty of mind remain. We have a Lord Who is both strict and indulgent. The prophet indeed said: “Look not upon the beauty of a woman that is all harlot.” But the Lord said: “Whoever shall look on a woman to lust after her, has committed adultery with her already in his heart.” He does not say, “Whosoever shall look has committed adultery,” but “Whosoever shall look on her to lust after her.” He condemned not the look but sought out the inward affection. But that modesty is praiseworthy which has so accustomed itself to close the bodily eyes as often not to see what we really behold. For we seem to behold with the bodily sight whatever meets us; but if there be not joined to this any attention of the mind, the sight also, according to what is usual in the body, fades away, so that in reality we see rather with the mind than with the body.
71. And if the flesh has seen the flame, let us not cherish that flame in our bosoms, that is, in the depths of the heart and the inward part of the mind. Let us not instil this fire into our bones, let us not bind bonds upon ourselves, let us not join in conversation with such as may be the cause to us of unholy fires. The speech of a maiden is a snare to a youth, the words of a youth are the bonds of love.
72. Joseph saw the fire when the woman eager for adultery spoke to him. She wished to catch him with her words. She set the snares of her lips, but was not able to capture the chaste man. For the voice of modesty, the voice of gravity, the rein of caution, the care for integrity, the discipline of chastity, loosed the woman's chains. So that unchaste person could not entangle him in her meshes. She laid her hand upon him; she caught his garment, that she might tighten the noose around him. The words of a lascivious woman are the snares of lust, and her hands the bonds of love; but the chaste mind could not be taken either by snares or by bonds. The garment was cast off, the bonds were loosed, and because he did not admit the fire into the bosom of his mind, his body was not burnt.
73. You see, then, that our mind is the cause of our guilt. And so the flesh is innocent, but is often the minister of sin. Let not, then, desire of beauty overcome you. Many nets and many snares are spread by the devil. The look of a harlot is the snare of him who loves her. Our own eyes are nets to us, wherefore it is written: “Be not taken with your eyes.” So, then, we spread nets for ourselves in which we are entangled and hampered. We bind chains on ourselves, as we read: “For every one is bound with the chains of his own sins.”
74. Let us then pass through the fires of youth and the glow of early years; let us pass through the waters, let us not remain therein, lest the deep floods shut us in. Let us rather pass over, that we too may say: “Our soul has passed over the stream,” for he who has passed over is safe. And lastly, the Lord speaks thus: “If you pass through the water, I am with you, the rivers shall not overflow you.” And the prophet says: “I have seen the wicked exalted above the cedars of Libanus, and I passed by, and lo, he was not.” Pass by things of this world, and you will see that the high places of the wicked have fallen. Moses, too, passing by things of this world, saw a great sight and said: “I will turn aside and see this great sight,” for had he been held by the fleeting pleasures of this world he would not have seen so great a mystery.
75. Let us also pass over this fire of lust, fearing which Paul— but fearing for us, inasmuch as by buffeting his body he had come no longer to fear for himself— says to us: “Flee fornication.” Let us then flee it as though following us, though indeed it follows not behind us, but within our very selves. Let us then diligently take heed lest while we are fleeing from it we carry it with ourselves. For we wish for the most part to flee, but if we do not wholly cast it out of our mind, we rather take it up than forsake it. Let us then spring over it, lest it be said to us: “Walk in the flame of your fire, which you have kindled for yourselves.” For as he who “takes fire into his bosom burns his clothes,” so he who walks upon fiery coals must of necessity burn his feet, as it is written: “Can one walk upon coals of fire and not burn his feet?”
76. This fire is dangerous, let us then not feed it with the fuel of luxury. Lust is fed by feastings, nourished by delicacies, kindled by wine, and inflamed by drunkenness. Still more dangerous than these are the incentives of words, which intoxicate the mind as it were with a kind of wine of the vine of Sodom. Let us be on our guard against abundance of this wine, for when the flesh is intoxicated the mind totters, the heart wavers, the heart is carried to and fro. And so with regard to each that precept is useful wherein Timothy is warned: “Drink a little wine because of your frequent infirmities.” When the body is heated, it excites the glow of the mind; when the flesh is chilled with the cold of disease the spirit is chilled; when the body is in pain, the mind is sad, but the sadness shall become joy.
Source: On Repentance (New Advent)