Question: R. Elazar and R. Shimon ben Gamliel say the same thing!

Suggestion: R. Elazar says that even one person can challenge his lineage, and R. Shimon ben Gamliel requires two.

Rejection: R. Yochanan taught that all agree that two are needed to challenge.

Answer #1: Rather, they argue in a case when two challenged.

The case is, we know that David's father is a Kohen. There were rumors that David's mother was divorced or a Chalutzah,

Beis Din put David's status in question until it would be researched. One witness said that David is a valid Kohen, and he was re-established to be a Kohen.

Two witnesses then came and said that his mother was divorced or a Chalutzah, so Beis Din declared that he is not a Kohen. Another witness came and said that David is a Kohen.

All agree that the first and last witnesses, in general, can join. They argue about whether here we are concerned for disgrace to Beis Din.

R. Elazar is concerned. Since Beis Din disqualified him, they do not reinstate him;

R. Shimon ben Gamliel says, they reinstate him. We are not concerned for disgrace to Beis Din.

Objection (Rav Ashi): The Mishnah implies that two witnesses always suffice to establish someone to be a Kohen;

If R. Elazar is concerned for disgrace to Beis Din, even if two witnesses come together (after others disqualified him), we would not reinstate him!

Answer #2 (Rav Ashi): Rather, all agree that we are not concerned for disgrace to Beis Din;

They argue about when we can join witnesses, like R. Noson and Chachamim argue (in the end of this Beraisa):

(Beraisa): Two witnesses cannot testify together unless they saw the testimony together;

R. Yehoshua ben Korchah says, even if they saw the testimony at different times, they join.

We cannot join the testimony of two witnesses unless they testify together;

R. Noson says, they can testify at different times.

Reuven challenged Shimon 'what are you doing on my field?' Shimon said 'I bought it from you. Here is the document.'

Reuven: That is a forgery!

Shimon whispered to Rabah (who was judging the case) 'indeed, it is a forgery, but I had a proper document and I lost it.' (The case is, Shimon could have validated the document.)

Rabah: Shimon is believed. Had he wanted to lie, he could have insisted that it is a proper document!

Rav Yosef: We cannot rule in Shimon's favor due to the document, for now we know that the document is a forgery!

Levi asked Yehudah to pay the money he owes, and showed the loan document.

Yehudah: That is a forgery!

Levi whispered to Rabah 'indeed, it is a forgery, but I had a proper document and I lost it.'

Rabah: Levi is believed, for he could have insisted that it is a proper document!

Rav Yosef: We cannot rule in his favor due to the document, for now we know that it is a forgery!

(Rav Idi bar Avin): The Halachah follows Rav Yosef in a dispute over money. The Halachah follows Rabah in a dispute over land. (Rashbam - we are in doubt whom the Halachah follows, so we leave the property where it is; Tosfos - we apply Migo only to keep property, but not to take it.)0

The Halachah follows Rabah in a dispute over land. Shimon is now on the land, we leave him there;

The Halachah follows Rav Yosef in a dispute over money. We leave the money where it is (with Yehudah).

Reuven, the Arev (guarantor) for a loan, told Shimon (the borrower) 'I paid up your loan. Here is the document. Pay me back.'

Shimon: I already paid you back!

Reuven: You borrowed the money back again!

Shimon denied this.

Question (Rav Idi bar Avin): What is the law?

Question (Abaye): Why do you ask? You yourself said that the Halachah follows Rabah regarding land, and Rav Yosef regarding money! (This is a case of money, so) we leave the money where it is (with Shimon)!

This is only if Reuven claims that Shimon borrowed the money again. If Reuven says that he returned the coins because they were rubbed out or reddish (and he demands to receive good coins), the document is still in force, and Shimon must pay.