The reason is that it is assumed that the priest-shepherd would say: My Israelite employer will not forsake a priest who is a Torah scholar and give the blemished animal to me, an unlearned shepherd. The case of priest-shepherds is where the shepherds are Israelites in the employ of a priest. These shepherds are not deemed credible to testify, as we are concerned that they might be lying for a swallow of the firstborn that their priest employer would give them in exchange.

And all the more so the testimony of a priest for the sake of another priest is not deemed credible, as we are concerned that the two might engage in reciprocal behavior, and we are concerned that the priest-shepherd might be lying for a swallow of the firstborn offering. And Rabban Shimon ben Gamliel comes to say that the priest-shepherd is deemed credible to testify with regard to the firstborn offering of another priest, but is not deemed credible to testify with regard to a firstborn belonging to himself. And Rabbi Meir comes to say that a priest who is suspect about the matter of causing a blemish may neither adjudicate nor testify in cases involving that matter, even on behalf of another.

The Gemara raises a difficulty: Granted, according to the second opinion, i.e., the one who says the first tanna maintains that the case of Israelite shepherds is where the shepherds are priests in the employ of an Israelite, and they are deemed credible to testify, this is the reason that Rabbi Meir comes to disagree and say that a priest who is suspect about the matter of causing a blemish may neither adjudicate nor testify even for the sake of an Israelite employer.

But according to the first opinion, i.e., the one who says that the first tanna maintains that the case of priest-shepherds is where the shepherds are in an Israelite’s house, and they are not deemed credible to testify, what is Rabbi Meir coming to teach us? This is the same opinion as that of the first tanna himself.

The Gemara answers: There is a difference between the opinions of the first tanna and Rabbi Meir with regard to a statement of Rabbi Yehoshua ben Kefusai. As it is taught in a baraita that Rabbi Yehoshua ben Kefusai says: If a firstborn animal in the house of a priest developed a blemish, two people from the marketplace, i.e., not from the priest’s household, are required to testify about it that the blemish was not caused deliberately. This unqualified statement indicates that the two people can even be priests, provided that they are not household members.

The baraita continues: Rabban Shimon ben Gamliel says: Even the priest’s son and even his daughter may testify about it. Rabbi Yosei says: Even in a case where there are ten people, if they are members of his household, they may not testify about it. The first tanna of the mishna maintains that only a priest who is a shepherd of the animal in question is not deemed credible to testify, but the testimony of an independent priest is credible. This accords with the opinion of Rabbi Yehoshua ben Kefusai, who allows the testimony of any two independent people, including a priest. Conversely, Rabbi Meir does not deem credible even the testimony of an independent priest, as indicated by his general statement about suspect priests.

The Gemara discusses a related statement: In accordance with whose opinion is that which Rav Ḥisda says that Rav Ketina says: In the case of an animal whose status as a firstborn is uncertain that was born in the house, i.e., in the possession, of an Israelite, e.g., it was uncertain whether the mother had previously given birth, in which case the animal remains in the possession of the Israelite and may be eaten upon developing a blemish, two people from the marketplace are required to testify about it?

In accordance with whose opinion is this statement? It is in accordance with the opinion of Rabbi Yehoshua ben Kefusai. Just as he requires two independent people to testify about a firstborn offering in the possession of a priest, as he is suspected of intentionally causing the blemish, so too, he requires two independent people to testify in the case of an uncertain status of a firstborn offering born in the possession of an Israelite.

Rav Naḥman disagrees and says: The Israelite owners themselves may testify about it. As, if you do not say so, but instead maintain that any involved person is suspected of deliberately causing a blemish in his firstborn animal, this poses a difficulty with regard to the halakha of an animal tithe offering, which may also be eaten if it developed a blemish. According to Rabbi Meir, who can testify about it? Rabbi Meir maintains that anyone who is suspect about the matter of causing a blemish on his own behalf may neither adjudicate nor testify in cases involving that matter even on behalf of another. If so, how can anyone, even an Israelite, testify about any blemished animal-tithe offering? It must be that an Israelite is not suspected of deliberately causing blemishes.

The Gemara rejects this claim: With regard to the animal tithe offering, the owner is certainly deemed credible to testify that its blemish occurred naturally, because if he wants, he could legitimately cause a blemish in his entire flock before the obligation to separate tithes came into effect. Rather, this is what Rav Naḥman stated: If an Israelite is not deemed credible to testify about a blemish found on an animal whose status as a firstborn is uncertain, then according to Rabbi Meir, who can testify about it? Both Israelites and priests stand to gain if their animal whose status as a firstborn is uncertain develops a blemish, and therefore no one should be deemed credible to testify about it.

And if you would say that indeed, according to Rabbi Meir, an animal whose status as a firstborn is uncertain has no remedy that can render it fit for slaughter, as no one is deemed credible to testify about its blemish, this cannot be correct. But didn’t we learn in a mishna (18b) that Rabbi Yosei would say: Any animal whose replacements are in the possession of a priest is exempt from, i.e., not subject to, the mitzva of giving the priestly gifts, and Rabbi Meir deems him obligated to give the gifts? Since Rabbi Meir permits the consumption of an animal whose status as a firstborn is uncertain, evidently he allows testimony about its blemish in such a case.

Rather, learn from this that the Israelite owners may testify about their animals whose status as a firstborn are uncertain, despite the fact that priests may not. The reason is that it is only priests who are suspected of causing blemishes; Israelites are not suspected of causing blemishes.

§ It was stated: Rav Naḥman says that the halakha is in accordance with the opinion of Rabban Shimon ben Gamliel, that a priest is deemed credible to testify about the blemished firstborn animal of another priest, even if he is a household member. Rava says: The halakha is in accordance with the opinion of Rabbi Yosei, that one’s household members are not deemed credible to testify.

The Gemara asks: And does Rava actually say this? But doesn’t Rava say: If the priestly owner of a firstborn animal was standing outside with us, and the animal entered the house whole and emerged injured, the household members may testify about it that the blemish was not caused by a person. Evidently, Rava maintains that the members of the priest’s household are deemed credible to testify about the priest’s firstborn. The Gemara answers: One can say that Rava is referring to a case where all the owners, i.e., the household members, are standing outside, while the shepherd alone remains inside. When the firstborn animal emerges injured, the shepherd is deemed credible to testify about the blemish and we are not concerned that he is lying.

The Gemara asks: If so, that all of the household members were outside, what is the purpose of stating that the shepherd is deemed credible? This ruling is obvious. The Gemara answers: This ruling is necessary lest you say that we should entertain a suspicion that the shepherd himself caused the blemish. Rava therefore teaches us that this is not so, and the shepherd is deemed credible. The Gemara concludes: And the halakha is in accordance with the opinion of Rabban Shimon ben Gamliel, that a priest’s household members may testify about his blemished firstborn offering. And this applies specifically to his son and daughter, but his wife may not testify. What is the reason? One’s wife is as oneself.

§ Rav Pappa said to Abaye: According to Rabbi Meir, who says that a priest who is suspect about the matter of causing a blemish may neither adjudicate nor testify in cases involving that matter even on behalf of another, and in addition Rabbi Meir says that one who is suspected of transgressing one matter, i.e., who is known to have committed one transgression, is suspected of transgressing the entire Torah, it should follow that priests should also not be allowed to administer judgment at all. But isn’t it written with regard to the priests: ''And according to their word shall every controversy and every stroke be'' (Deuteronomy 21:5)?