Bechorot, Chapter 1, Mishnah 5. In the previous Mishnah we learned that Kohanim are suspected of improper conduct regarding inflicting a blemish on a firstborn animal. However, in practice, there are two possible types of improper conduct here:
The lighter of the two: Inflicting a blemish on the animal intentionally so that it can be slaughtered - and of this, Kohanim are indeed suspected.
The more severe of the two: Not bothering at all with the first step, but rather slaughtering a firstborn animal before an authorized person has declared that it is permitted to be slaughtered - and of this, a kohen is not suspected. As long as the animal has the status of being unblemished, without a defect, or it has not yet been determined with certainty that it has sustained a blemish, it is considered consecrated property (kodashim), and slaughtering it outside the Temple is a severe transgression punishable by karet - spiritual excision by Heaven, similar to eating on Yom Kippur and desecrating Shabbat.
It turns out that Kohanim are not suspected of causing animals to be slaughtered before they are declared blemished. The only concern is that they might plan to cause them to develop a permanent blemish by inflicting a wound.
"Ne'eman hakohen lomar":
Therefore, the Mishnah states: "Ne'eman hakohen lomar: her'eiti bechor zeh uva'al mum hu" - The kohen is believed to state, whether to the slaughterer or to the buyer of the animal, that he has already shown it to an expert and it was confirmed to be a blemish, and we believe him regarding this.
The Gemara brings two reasons for this credibility:
A kohen is not suspected of causing himself and others to stumble by slaughtering consecrated animals outside the Temple.
The matter is considered a "milta de'avida le'igluyei" - something that is destined to be revealed eventually, and the truth will come to light. If the kohen claims that he showed the animal to an expert, the expert might contradict him and say that he never saw his animal, and the kohen will be established as a liar - a more severe consequence that a person tries to avoid. From here we derive a general halachic presumption: in matters where the truth is destined to be revealed, we do not assume that a person will lie. This principle will return to us in practical halachah, and it has ramifications.
"Hakol ne'emanin al mumei hama'aser":
The Mishnah continues: "Hakol ne'emanin al mumei hama'aser" - Everyone is believed regarding blemishes of an animal tithe. Although this is a tangential remark that is not directly related to our topic, it complements what was said in the previous Mishnah: we suspect Kohanim of deceit because the firstborn that comes into their hands constitutes a massive financial burden until it sustains a blemish, and therefore they have a strong interest in seeing it blemished as early as possible.
The animal tithe applies to everyone - kohen, Levite, or Yisrael. Someone who has ten animals in his flock separates a tithe from them every year, and the tenth animal that emerges from the corral has the same status as a firstborn in every respect: it is consecrated and requires sacrifice in the Temple. In the absence of a Temple, or when the owners are located outside the Land of Israel and cannot bring it, they must leave it to graze until it sustains a blemish on its own, and only then may they slaughter and eat it. Just as the firstborn can be a heavy financial burden for years without sustaining a blemish, so too the tithe - the farmer is required to feed, sustain, and care for it for years upon years.
Seemingly, just as we suspect a kohen of inflicting a blemish intentionally in order to render the animal blemished, we should have suspected every farmer who has tithes. However, the Mishnah states that they are not suspected, and the reason for this is: someone who wishes to bypass the laws of the animal tithe from the outset, to slaughter and eat them immediately without bringing sacrifices, has an easy and simple path before him - to inflict a blemish on the animals even before taking them out of the corral one by one to designate the tenth as a tithe. For up until that moment, all of his animals are unconsecrated (chullin), and there is no restriction whatsoever against slitting the ear of an unconsecrated animal; this does not constitute harming consecrated property, and therefore it is not a problem.
One who wants to cheat the system could have, therefore, inflicted a blemish on all of his animals in advance, brought them into the corral, and taken them out one by one, and when the tenth emerges, it will already be blemished and he can eat it immediately. Because of this, a person is not suspected of lying in matters of tithes and cheating by inflicting blemishes, since he could have planned this in a completely permissible manner anyway if he had wanted to.
"Bechor shensameis eino":
Here the Mishnah returns to matters of the firstborn and kohanim: "Bechor shensameis eino, shenikteah yado, shenishberah raglo" - a firstborn animal whose eye was blinded, whose front leg was cut off, or whose hind leg was broken and crushed. All of these are blemishes, and it is clear that this is a permanent blemish: there is no chance that a crushed leg, a missing leg or a blind eye will heal. Therefore, technically there is no need for an expert to identify this, and as the Mishnah says: "harei zeh yishacheit al pi sheloshah bnei haknesses" - it is sufficient for three people from the synagogue to rule that this is indeed a blemished animal.
The expression "bnei haknesses" is unusual, and its meaning is twofold:
On the one hand: They do not need to be actual experts; any person who comes through the doors of the synagogue is sufficient.
On the other hand: These are not complete ignoramuses who sit in a corner playing backgammon all day, but people who are regulars at the synagogue, who listen to words of Torah, the rabbi's classes or Daf Yomi. They are not rabbis, but they understand a thing or two, and they have the ability to join a temporary rabbinical court of three and say: approved, it is permissible to eat this animal, because it has an obvious blemish - a missing eye or a missing leg.
The opinion of Rabbi Yosi:
Rabbi Yosi disagrees and holds that there is no permission except by an expert alone. Even a court of twenty-three, which is fit to sit as a minor Sanhedrin and judge capital cases and execute people - if it does not have the ordination of the Sanhedrin or the Nasi in the Land of Israel, it is not authorized to judge firstborns. The only one who can permit a firstborn for consumption is a certified expert, who possesses the expertise and ordination from the Nasi or the Sanhedrin.
The halachah does not follow Rabbi Yosi but rather the Tanna Kamma. A person who owns a firstborn and has not entered into a partnership with a non-Jew - the firstborn is forbidden for consumption. Once it develops a blemish it is permitted for consumption, but three of the bnei haknesses must be convened, people who are not ignoramuses and understand a thing or two, in order to confirm that it has indeed become blemished, and only then may it be eaten.
Returning to the credibility of the kohen:
It is interesting to return to the beginning of the Mishnah, which states that the kohen is believed to say he showed the animal to an expert who permitted it. Specifically with an obvious blemish, the kohen is surprisingly more suspect, because he might say to himself that there is no need to bother showing it, since it is plainly visible that the animal has only three legs. However, the mere fact that the blemish is recognizable is not enough - a ruling is required that the animal is blemished. Because of this, when dealing with a prominent and obvious blemish that anyone can see, and the kohen says that he convened three of the bnei haknesses to permit the animal, he must name the expert to whom he showed it. Once he names him, the matter becomes a "milsa de'avida le'igluyei" - a matter that is bound to be discovered, and therefore he is believed. But if he does not say to whom he showed it, he is not believed, since we are concerned that he skipped this step and did not act according to the law.
In conclusion: In this Mishnah we learned that the kohen is believed to say that he showed his firstborn to an expert and it was recognized as blemished - both because he is not suspected of slaughtering holy sacrifices outside and incurring kares, and because it is a "milsa de'avida le'igluyei". We also learned that everyone is believed regarding blemishes of the tithe, since the owner could have inflicted a blemish on his animals completely permissibly ahead of time; and that a firstborn that developed a permanent and obvious blemish is permitted by three of the bnei haknesses - and according to the opinion of Rabbi Yosi by a certified expert alone, and the halachah does not follow him.