In the Talmud Berachos 12b, Chazal interpret the meaning of “straying after one’s heart and eyes.” “Straying after the heart” is a reference to entertaining heretical thoughts, and “straying after the eyes” alludes to permitting lewd thoughts to enter one’s mind. The Torah enjoins us to distance ourselves from places or situations which will inspire such sinful contemplations. We are provided with the mitzvah of Tzitzis as a reminder of our obligation to exercise care in avoiding these spiritual hazards.
The Talmud Gittin 55b relates the story of Kamtza and Bar Kamtza, two people in Yerushalayim with very similar names. A third man who was close friends with Kamtza sent a messenger to him, inviting him to a feast he was planning. By mistake, the messenger invited Bar Kamtza, who was the host’s enemy. On the day of the party, when Bar Kamtza walked in, the host became enraged and ejected him from his house. Bar Kamtza tried everything to ward off the humiliation, but to no avail. He decided that since many members of Yerushalayim’s rabbinic leadership had witnessed his humiliation and did nothing to prevent it, he would stir up trouble for the entire Jewish religious community. He went to Caesar and told him that the Jews were initiating a rebellion against him. To prove it, he suggested that Caesar send a sacrifice to the Bais HaMikdash and see if they would accept it. Caesar sent a healthy calf to be offered in the Bais HaMikdash. Bar Kamtza, who was the agent appointed to bring the animal to Yerushalayim, made a blemish in the calf, thus insuring that the Kohanim would reject it. The rest is history. This was the precursor of the siege that led to the destruction of the Bais HaMikdash.
Horav Yosef Chaim Sonnenfeld, z.l., observes an allusion to the concept of “guarding one’s eyes” in the incident of Kamtza and Bar Kamtza. The blemish that Bar Kamtza inflicted was a small slit in the eyelid of the animal, which Chazal tell us is considered a blemish in regard to a korban, but is not considered a blemish by the gentiles in regard to their sacrifices. This was specifically why Bar Kamtza chose this type of blemish.
Rav Yosef Chaim interprets Chazal figuratively. The blemish in the eye signifies one’s lack of control over his eyes. This constitutes a moral flaw for a Jew and is, hence, a blemish. The non-Jewish nations do not view a roving eye as a character flaw. Indeed, it is a way of life for them.