[אור החיים - ויקרא יח, ג]

הנה ידוע הוא כי כל מצות אשר צוה ה' לעם קדושו הם מצות שיכול האדם לעמוד בהם ויטה עצמו אל הרצון לעשותם, זולת מצות פרישת העריות הוא דבר שנפשו של אדם מחמדתן ואונסתו עליהם לעשותם, זולת בהתעצמות הרחקת ב' דברים מהאדם, והם מרחק הרגש ראות העין, ומרחק בחינת החושב,

ואם ב' אלו לא יעשה - אין אדם שליט ברוח זה לכלותה ממנו, כי כל שלא תהיה לו הרחקת הרגש הראות בדבר - הגם שירחיק בחינת החושב - לא ישלוט בעצמו -  לכלות ממנו הכרח החשק.

וצא ולמד (קדושין פא.) ממעשה של ר''ע רבן של חסידים שהגם היותו מושלל מבחינת החושב נתנצח מבחינת הראות, - גם ממעשה רבי מתיא ב''ח (ילקוט שמעוני פ' ויחי) שבחר לסמות עיניו למיחוש הכרחה אשר מהם תבא בדבר הרע - הגם שהיה מושלל מבחינת החושב - כאשר מעשיו מוכיחות.

גם אם יושלל מבחינת הראות-  אם לא ירחיק בחינת החושב מחשבותיו יחייבו לבקש ולהתלהט אחר המעשה.

וצא ולמד ממעשה שהובא בש''ס (ע''ז כב:) באותו גוי שקנה ירך בשר וחטט בו כדי וכו' ובעל וכו' ע''כ. וזהו יהיה תולדות החושב הכריחתו עשות מבלי בחינת הראות.

נמצאת אומר שבא' מהב' תהיה מושללת מהאדם שליטה בעצמו בדבר זה. ואין צריך לומר בהצמד ב' דברים ראות וחושב הן האדם חלוש כנגד תאותו, ואין צריך לומר אם ישלח ידו ואכל מעט מן הרע הזה - הנה הוא מסור ביד תאותו.

מעתה תחבולות האדם להעריך מלחמה נגד בחינה זו היא בשלילת ממנו ב' דברים ובזה תהיה נרכבת באדם תכונה לבל יתלהט אחר זה וישלוט הרצון בחפץ הטבעי והוא טעם אמרו ז''ל (ברכות לד:) במקום שבעלי תשובה עומדים אין צדיקים גמורין יכולין לעמוד, כי הצדיקים גמורים תכונת רצונם נוצחת החפץ מבלי צורך התעצמות מה שאין כן בעלי תשובה:

It is well known that all the mitzvos that Hashem issued to His holy nation are mitzvos whose challenges a person can withstand, by directing himself to act according to his higher will, which is to fulfill the mitzos, and to reject his physical desires that may conflict with a mitzvah.

This is with the exception of the mitzvah to abstain from immoral acts, for it is something that man's instinctive nature craves, and this craving practically compels him to engage in them.

The only way for a person to prevent himself from sinning, is by making every effort to distance the two means of approach to immorality; these are: (1) he must avoid immorality with the sense of sight, ie., he should not look at anything inappropriate; and (2) he must avoid the component of sinful thought, i.e., he must never focus on matters of immorality.

If one does not do both of these things, one cannot rule over this impulse toward immorality and rid himself of it. For as long as he does not distance his sight from seeing immoral things, even if he avoids the component of sinful thought and does not focus on such things, he will not be able to control himself and will not be able to rid himself of the compelling desire.

You can learn this from the incident recorded in the Gemara (Kiddushin 81a) that occurred with Rav Amram, the master among pious men, who, although entirely removed from immorality in terms of his thoughts, was nevertheless overcome by the component of sight. 

You also see this from the incident with R' Masya ben Charash (Yalkut Shimoni 161), who chose to blind his eyes out of concern that the urge generated by the sight of [his eyes] would bring him to commit a sin, even though he was certainly free of the component of immoral thought, as is evident from his actions.

 Conversely, even if a person should be free of the component of immoral sight, if he does not also avoid the component of immoral thought, his thoughts will compel him to seek and be driven to the act. You can learn this from the incident related in the Gemara (Avodah Zarah 22b)

regarding a certain idolater who purchased a meat-thigh, carved it, and "cohabited" with it.

It emerges from this that through either of these two things, i.e., seeing tempting sights or engaging in sinful thoughts, a person will lose his self-control with regard to this matter.

And it goes without saying that when these two things, seeing tempting sights and thinking sinful thoughts, are joined together, a person is certainly too weak to fight against his desire, and will succumb to it. And it certainly goes without saying, that if a person has stretched out his hand and "eaten" even a bit of this forbidden “food,”  i.e., he has once succumbed to this temptation, with that sinful act, he becomes controlled by his desire, and it is especially difficult for him to avoid immorality again.

Thus, the strategy a person must employ to wage battle against this kind of temptation is composed of completely avoiding both of these things, immoral sights and immoral thoughts. 

In this way the person can develop the nature not to be I driven toward this sin, and he will enable his higher will, which is to comply with Hashem's commandment, to control his lower natural craving for this sin.

This idea, that once one is exposed to this sin, he becomes strongly drawn to it, is the basis for the statement of our Sages], of blessed memory (Berachos 34b), that in the exalted place where penitents stand, even the completely righteous cannot stand; that is, in one sense, those who have sinned and repented are greater than those who have never sinned. This is because the higher will of the completely righteous overcomes the instinctive craving for immorality without a need for intense effort on their part, since they are removed from the experience of immorality, in action as well as in sight or thought. This is not so for those who have sinned and repented, they are forced to constantly fight against their natural craving, which is intensified due to their past sins. For them to conquer their desires requires much greater effort than it does for the completely righteous.