(Beraisa - Rebbi): If Reuven says that he has a document and a Chazakah for his land, he must produce the document;

R. Shimon ben Gamliel says, he brings witnesses of Chazakah.

Question: What do they argue about?

Answer #1 (Rav Dimi): They argue about whether or not Osiyos are acquired through Mesirah.

R. Shimon ben Gamliel says that Osiyos are not acquired through Mesirah (until he also makes Chazakah in the land);

Rebbi says that Osiyos are acquired through Mesirah.

Objection (Abaye): This argues with Rabah (he says that R. Shimon ben Gamliel holds that Osiyos are acquired through Mesirah)!

Rav Dimi: Yes, I argue!

Abaye: The only way to explain the Mishnah is like Rabah!

Answer #2 (Abaye): The case is, one of the witnesses was found to be a relative or invalid witness. They argue as R. Meir and R. Elazar argue;

Rebbi holds like R. Elazar, who says that Edei Mesirah Karsei (a document is empowered by the witnesses who saw it given. Therefore, as long as valid witnesses saw the document given, it is valid);

R. Shimon ben Gamliel holds like R. Meir, who says that Edei Chasimah Karsei (a document is empowered by the witnesses who signed it. Therefore, the document is invalid).

Rejection: R. Aba taught that R. Elazar admits that a document signed by invalid witnesses is invalid!

Answer #3 (R. Avina): All agree that if the document says 'we (Beis Din) investigated the testimony, and we found that it is invalid', the document is invalid, as R. Aba taught;

They argue about a document without signatures.

Rebbi holds like R. Elazar, that Edei Mesirah Karsei (therefore, it is valid without signatures);

R. Shimon ben Gamliel holds like R. Meir, who says that Edei Chasimah Karsei (therefore, it is invalid).

Answer #4 (R. Avina): They argue about whether or not the lender must validate a document if the borrower admits that he authorized it (but claims that he paid it):

Rebbi says, he need not validate it;

R. Shimon ben Gamliel says, he must validate it. (If not, the borrower is believed to say that he paid, Migo (since) he could claim that it was forged.)

Question: They hold just the contrary!

(Beraisa - Rebbi): Reuven (the lender) and Shimon (the borrower) were fighting over the document: Reuven says 'I dropped it, and you found it.' Shimon says 'no, I paid you.' Reuven must validate it to collect;

R. Shimon ben Gamliel says, he collects half.

Question: Does Rebbi argue with the following Mishnah?!

(Mishnah): If Levi and Yehudah were holding a garment, and each claims 'I found it', each swears that he does not own less than half, and they divide it. (Here also, perhaps Shimon dropped it. Why can Reuven collect the full amount if he validates it?)

Answer (Rava): If the document was validated, all agree that he collects half;

They argue in a case that when it was not validated. Rebbi says, even though Shimon admits that it he authorized it, Reuven must validate it (to collect half);

R. Shimon ben Gamliel says, he need not validate it.

Answer #1: We must switch the opinions.

Answer #2: We need not switch the opinions. They argue about whether one must justify all his claims:

Rebbi says that he must. He must validate the document and bring witnesses of Chazakah;

R. Shimon ben Gamliel says that one need not justify all his claims. It suffices to bring witnesses of Chazakah.

Rav Yitzchak bar Yosef claimed that R. Aba owed him money.

R. Aba: I paid you in front of Ploni and Almoni.

R. Yitzchak Nafcha (who was judging the case): Let Ploni and Almoni come testify!

R. Aba: Am I not believed without them? If one borrowed money in front of witnesses (and no document was written), he need not pay back in front of witnesses!

R. Yitzchak Nafcha: You yourself taught in Rav's name that if one says 'I paid you in front of Ploni and Almoni', they must testify for him!

R. Aba: Rav said that the Halachah follows R. Shimon ben Gamliel. Even Rebbi said to validate the document only in order to clarify his claim (but if he cannot, he does not lose).

R. Yitzchak Nafcha: Also I said only that they should testify in order to clarify your claim!

(Mishnah - R. Yehudah): If Reuven paid part of his debt to Shimon, he exchanges (tears the document and writes a new document for the remainder);

R. Yosi says, Shimon gives Reuven a receipt for what he paid.

R. Yehudah: If so, Reuven must guard his receipt, lest mice eat it (and Shimon will force him pay the entire debt)!

R. Yosi: That is better than making Shimon lose his lien (from the time of the original document).

(Gemara - Rav Huna citing Rav): The Halachah does not follow R. Yehudah, nor R. Yosi;

Rather, Beis Din tears the old document, and write a new document from the original date.

Rav Nachman: Had Rav heard the following Beraisa, he would have retracted!

(Beraisa): Witnesses tear the old document, and write a new document from the original date.

Rav Huna: Rav heard it, but he did not retract. (He is a Tana. He may argue with a Beraisa.)

Beis Din has authority to dispossess money as they see fit (to write a predated document);

The witnesses did what they were told (to write a document from the original date). They have no authority to do so again!