1. One is believed to say, “This is a bechor and this is its מום” (הפה שאסר הוא הפה שהתיר)

רבי אילעא said that if an animal was not known to be a bechor, the owner (a Kohen) is believed to say שהוא בכור ומומו עמו – that it is a bechor and its permitting blemish is with it, i.e., its מום occurred naturally. The Gemara objects: is רבי אילעא teaching us שהפה שאסר הוא הפה שהתיר – that the mouth which forbade is the mouth which permitted, i.e., since its being a bechor is only known through his admission, his second statement is also believed? But this principle was already taught in a Mishnah: a woman who said אשת איש הייתי וגרושה אני – “I was married but now I am divorced” is believed, because הפה שאסר הוא הפה שהתיר (since we did not know she was married without her admission)!? The Gemara answers that in that case, she could have said nothing and been presumed unmarried. Here, the Kohen must inform us it is a bechor, because he would not eat kodashim outside the Mikdash, and needs the sage to examine its מום. רבי אילעא teaches that he is believed, because had he inflicted the מום himself, he would have inflicted an obvious מום and eaten it without showing it to a sage.

1. One is believed to say, “I showed this bechor to a sage and it is a בעל מום”

The next Mishnah states that a Kohen is believed to say: הראיתי בכור זה ובעל מום הוא – “I showed this bechor to a sage, and he ruled it is permanently blemished.” He may then shecht it (but must provide testimony that the מום occurred naturally). Rav said a Kohen is believed to say: בכור זה נתן לי ישראל במומו – “A Yisroel gave me this bechor with its מום,” and does not require testimony that he did not inflict it, because כל מילתא דעבידא לאיגלויי לא משקרי בה אינשי – people do not lie about what is likely to be revealed (i.e., since his lie would be exposed if the Yisroel were asked). Rav Ashi said our Mishnah supports this: one is believed to say he showed the מום to a sage, since the sage could be asked. The Gemara rejects the proof: in the Mishnah, he might be believed the מום was certified because he would not eat kodashim outside the Mikdash, but here, כיון דחשידי חשידי – since they are suspected of inflicting blemishes and claiming they occurred naturally, they are suspected of falsely claiming they received it from a Yisroel with a מום.

1. Obvious מומין can be certified by three laymen

The Mishnah teaches, regarding a bechor whose eye was blinded, its foreleg was cut off, or its hind leg was broken, הרי זה ישחט על פי ג' בני הכנסת – it may be shechted by the authorization of three laymen, even without a sage, since the מום is obvious to all. Rebbe Yose says that even if there are twenty-three judges, אינו נשחט אלא על פי מומחה – it may only be shechted by the authorization of an expert. Amoraim quote a ruling that a bechor in חוץ לארץ may be permitted by three laymen (even in the time of the Beis Hamikdash). Rava added that this is only true for מומין מובהקין – obvious blemishes. Rebbe Yirmiyah bar Abba quoted that three laymen may permit a bechor במקום שאין מומחה – in a situation where there is no expert, but if there is a מומחה, it must be shown to the מומחה.