Counsels to be Cautious
The Torah doesn't elaborate, לא תרצח, "Don't murder because you have the nature to do so". And the Torah doesn't say לא תחמוד, "Don’t covet what belongs to others, because you tend to stray after these matters". However, when it comes to the obligation of guarding one’s eyes, the Torah tells us that we tend to stray after this aveirah. As it states (15:39) וְלֹא תָתוּרוּ אַחֲרֵי לְבַבְכֶם וְאַחֲרֵי עֵינֵיכֶם אֲשֶׁר אַתֶּם זֹנִים אַחֲרֵיהֶם, "You shall not wander after your hearts and after your eyes after which you are going astray." This is the only mitzvah of the Torah, which the Torah warns us that we must be cautious with because we tend to stray and to perform this aveirah. The Torah instructs us to find solutions and techniques to guard our eyes, as it is very difficult to remain cautious. For all other aveiros of the Torah, it is sufficient to make a kabbalah tovah that you won't perform the aveirah. But when it comes to the aveirah related to the eyes, in addition to the kabbalah tovah, one has to make gedarim, boundaries, safeguards, to prevent him from committing the aveirah.
Segulah for Parnassah
Beis Avraham teaches that guarding the eyes is mesugal for parnassah, because it states אַחֲרֵיהֶם זֹנִים אַתֶּם רַאֲש, and זֹנִים is from the word מזונות, parnassah.
It states (Yeshayahu 33:15-16), וְעֹצֵם עֵינָיו מֵרְאוֹת בְּרָע... מְרוֹמִים יִשְׁכֹּן מְצָדוֹת סְלָעִים מִשְׂגָּבוֹ לַחְמוֹ נִתָּן מֵימָיו נֶאֱמָנִים, He who... closes his eyes from seeing evil, he will dwell on high; rocky fortresses shall be his defense; his bread shall be given to him, and his water is sure." Rashi writes, "[A person who guards his eyes] he won't seek bread, because heaven will give it to him... All of his needs will be given to him." This means guarding the eyes is mesugal for receiving parnassah and all one's needs.
It states (Bamidbar 11:7) וְהַמָּן כִּזְרַע גַּד וְעֵינוֹ כְּעֵין הַבְּדֹלַח, "And the manna was like coriander seed, and its appearance was like the appearance of crystal." The manna represents parnassah. The Torah tells us כְּעֵין הַבְּדֹלַח וְעֵינוֹ, when his eyes are crystal and pure (which means he is cautious with his eyes) he will receive manna (parnassah).
It states (Tehillim 145:15) עֵינֵי כֹל אֵלֶיךָ יְשַׂבֵּרוּ וְאַתָּה נוֹתֵן לָהֶם אֶת אָכְלָם בְּעִתּוֹ, "Everyone's eyes look to You with hope, and You give them their food in its time." The word ישברו can be translated as "break". The pasuk is saying that if a person breaks his eyes, which means he guards his eyes, he will have parnassah.
Guarding the Eyes and Memory
Guarding the eyes has other segulos, such as ensuring a good memory. Someone once came to the Magid of Brisk and asked for a segulah for improving his memory, as he felt that his memory had become very weak and was deteriorating all the time. The Brisker Magid replied, "Why do you need to seek a segulah? Every day, you say the segulah, as it states (15:39-40) וְלֹא תָתוּרוּ אַחֲרֵי לְבַבְכֶם וְאַחֲרֵי עֵינֵיכֶם... לְמַעַן תִּזְכְּרוּ, 'You shall not wander after your hearts and after your eyes... so that you shall remember.'" The Magid of Brisk writes this story in his sefer חזיון גיא (which is printed together with עינים מחזה), and he writes, "This is because looking at an aveirah, and thoughts of an aveirah, weaken the memory of a person. This is certain and all medical scholars agree upon it."
The Magid of Brisk presents a proof for this from Rav Yosef and Rav Sheshes of the Gemara, both of whom had a phenomenal memory for Torah. Rav Yosef was called Sinai (see Brachos 64a), and Rashi writes that this is because "he was an expert in all Braysos". The Gemara (Eiruvin 67a) also tells us that Rav Chisda was embarrassed before the great memory and expertise of Rav Sheshes. Now, Rav Yosef and Rav Sheshes were both blind. This suggests that when one doesn't contaminate his eyes, he will have a phenomenal memory.
The Four Colors of the Eye
Reishis Chachmah (Shaar HaKedushah ch.8, quoting Tikunei Zohar p.128a) teaches that the eye has four colors. (Although it appears that there are only three colors of the eye, there are some countries where people have four colors. Also, when one looks closely, one can see that around the iris, there is a thin ring, and it is of another color.) The white of the eye represents the oceans that surround the world. The color ring represents all the countries of the world. The next color, the iris, represents Yerushalayim. The pupil of the eye represents the Kodesh Kadoshim, the holiest place in the world. We understand from this that if a person isn't cautious with his eyes, he is contaminating the holiest places of the world. In contrast, we understand that if a person is careful with his eyes, he merits that the Shechinah will reside with him.
