Bava Metzia 5B

ותקנתא לתקנתא לא עבדינן.

Likewise, the halakha that if the defendant is suspect with regard to taking a false oath the plaintiff takes the oath and collects the money is also a rabbinic ordinance, and we do not institute one rabbinic ordinance upon another rabbinic ordinance. Therefore no oath is administered.

ותיפוק ליה דהוה ליה רועה! ואמר רב יהודה: סתם רועה פסול.

The reason cited for the lack of credibility of the oath of the shepherd is that he is guilty of robbery. The Talmud asks: But why not let R' Zeira derive that he is disqualified from testifying or taking an oath because he is a shepherd; and Rav Yehuda says that an ordinary shepherd is disqualified from testifying? A shepherd is presumed to be a robber since shepherds allow the animals under their care to graze in the fields of other people.

לא קשיא: הא דידיה, הא דעלמא. דאי לא תימא הכי, אנן חיותא לרועה היכי מסרינן? והא כתיב: ״לפני עור לא תתן מכשל״! אלא חזקה: אין אדם חוטא ולא לו.

The Talmud rejects this: This is not difficult. That case, where he is presumed a robber, is a case where he herds his own animals, and this case, where he is not presumed a robber, is a case where he herds animals that belong to others. As if you do not say so, if even one who herds the animals of others is presumably a robber, how do we give our animals to a shepherd? Isn’t it written: “Do not put a stumbling block before the blind” (Leviticus 19:14)? It is prohibited to cause others to commit a transgression. Rather, there is a presumption that a person sins only for his own benefit, and one would not commit robbery for the benefit of animals that are not his.