WHAT WE LEARN FROM EACH MISHNAH
Had we only learned the Mishnah (15b) in which R. Yehoshua admits (that we believe one who says 'this field was your father's and I bought it from him'), one might have thought that there the one who forbids can permit, because he stands to lose money (Tosfos - he is Muchzak ; Rashi - if he was not sure, he would not have admitted);
This would not apply to witnesses (18b, who say 'these are our signatures, but we were invalid witnesses at the time') since they do not stand to lose money.
Had we taught only about witnesses, one might have thought that we believe them because they will not gain from lying, but we do not believe one who helps himself;
Had we taught only these two Mishnayos, one might have thought that they are believed only regarding money, but not regarding Isurim, such as a married woman (who claims that she was divorced - 22a).
Question: Why must we teach (22a) 'I was captured but I am Tehorah'?
Answer #1: We need to teach that if witnesses come after they were (permitted to be) married, they do not leave (the permission).
Question: This is like Rabah bar Avin (23a), who says that this clause applies only to a captive;
R. Oshaya holds that this clause applies also to one who claims that she was divorced. How can he answer?
Answer #2: We need to teach the case of two women who were captured (23b).
Question: Why do we need this case?
Answer: One might have thought that we should suspect that they are conspiring to falsely testify to help each other. The Mishnah teaches that this is not so.
Question: Why do we need the case of two men (our Mishnah)?
Answer: We need to teach the argument of R. Yehudah and Chachamim.
THE CONCERN LEST TWO SCHEME TO HELP EACH OTHER
(Beraisa): If Reuven said 'I am a Kohen, and so is David', we rely on him to let David eat Terumah, but not regarding lineage (for David to marry);
If a third man is with them, and each Kohen has two others testifying that he is a Kohen, this establishes them to have proper lineage;
R. Yehudah says, even regarding Terumah Reuven is not believed unless there is a third man.
Question: This shows that R. Yehudah is concerned lest two lie to help each other, and Chachamim are not concerned for this. Elsewhere, they hold contrary to this!
(Mishnah): If two merchants (Levi and Shimon) entered a city, and Levi said 'my produce is new and my friend's is old. Mine is Tevel and my friend's is tithed', he is not believed;
R. Yehudah says, he is believed.
Answer #1 (Rav Ada bar Ahavah): The opinions (in the Beraisa) must be switched.
Answer #2 (Abaye): R. Yehudah is lenient regarding Demai (doubtfully tithed produce), because most people tithe.
Question (Rava): We must also resolve the contradiction in Chachamim!
Answer #3 (Rava): We resolve R. Yehudah like Abaye did. We resolve Chachamim like Rav Chama bar Ukva answered (a different question), 'he is holding tools of the trade'.
Here also, when he is holding tools of the trade we are concerned that he is lying to help his friend. (Levi will not sell his produce by degrading it. Perhaps they conspire, and in another city Shimon will degrade his own produce and praise Levi's!)
Rav Chama answered the following question:
(Mishnah): If a potter (who is careful about Taharah) left his pots and went to drink, the inner ones are Tehorim and the outer ones are Temei'im.
Contradiction (Beraisa #1): Both of them are Temei'im.
Answer (R. Chama bar Ukva): (The Beraisa discusses when) he has his tools of the trade, so everyone touches the pots (since they know that they are for sale).
Contradiction (Beraisa #2): Both are Tehorim.
Answer (R. Chama bar Ukva): (This is when) he does not have his tools of the trade.
Question: The Mishnah says that only the outer pots are Temei'im. What is the case?
Answer: The pots are close to a major road with obstacles to distance wagons from the walls on the sides. (Clothing of ignoramuses is Tamei, and it touches or enters the airspace of outer pots near the obstacles.)
Answer #4: R. Yehudah and Chachamim argue about whether or not we establish a person to have proper lineage based on seeing him eat Terumah. (R. Yehudah says that we do, so he always requires two witnesses to establish a person to be a Kohen to eat Terumah. However,. in a case where one witness suffices, he is not concerned lest two conspire to testify falsely for each other.)
WHAT ESTABLISHES A PERSON TO HAVE PROPER LINEAGE?
Question: Can we establish that a person has proper lineage based on a document?
Question: What is the case?
If it says, 'I, Ploni the Kohen, sign as a witness', no one testifies about him!
Answer: Rather, it says 'I, Ploni the Kohen borrowed... ', and witnesses signed below.
Do witnesses testify only about the loan, or about everything in the document?
Answer: Rav Huna and Rav Chisda argued. One said that documents establish lineage, and the other said that they do not.
Question: If we see a man Duchan (bless Birkas Kohanim), does this establish him to have proper lineage?
This does not depend on whether or not we establish lineage based on eating Terumah.
Perhaps the one who establishes based on eating does so because a Zar (non-Kohen) who eats Terumah is liable to death at the hands of Shamayim (so surely he must be a proper Kohen). For a Zar to Duchan is only an Isur inferred from an Aseh, perhaps he would agree that it is not a proof of proper lineage. Or, perhaps we do not distinguish;
Perhaps the other opinion does not establish based on eating because Terumah is eaten in private. Birkas Kohanim is done in public, so a Zar would not be so brazen to Duchan, so it is a proof. Or, perhaps we do not distinguish.
Answer: Rav Chisda and R. Avina argued. One said that Birkas Kohanim establishes lineage, and the other said that it does not.
Question (Rav Nachman bar Yitzchak): May we establish lineage based on Birkas Kohanim?
Answer (Rava): Rav Chisda and R. Avina argued about this.
Question (Rav Nachman bar Yitzchak): What is the final ruling?
Answer #1 (Rava): I know a Beraisa (which says that we do not).
(Beraisa - R. Yosi): Chazakah is great! "Among the Kohanim... they could not find their documents of lineage, and were spurned from Kehunah. Ha'Tirshasa told them that they will not eat Kodshei Kodoshim (Korbanos permitted only to Kohanim) until (Mashi'ach will come and) a Kohen (Gadol) will wear the Urim v'Tumim (to rule about them)";
He said that they keep their Chazakah. Just like they ate Terumah in Bavel, they may eat Terumah in Eretz Yisrael. (Likewise, they may continue to Duchan.)
If we establish lineage based on Birkas Kohanim, people would come to approve of the lineage of these disqualified Kohanim!
Objection (Rav Nachman bar Yitzchak): There it is different, since people see a weakness in their Chazakah (they may not eat Kodshei Kodoshim).
Support (for Objection): If we would not say so, how would the opinion that establishes lineage from eating Terumah address this concern?!