I don’t think that’s what he means, because at the beginning of the section he argues that the litigant is also defined as a relative, and nevertheless he does have the law of migo. So what is different from a witness? And therefore throughout the whole section he brings proofs that migo is a power of credibility and not the logic of “why would I lie,” and this law about the witness is one of the proofs for that. He also wants to explain, based on this distinction, the Rashbam’s view in the case of a forged document. I’d appreciate it if the Rabbi could go over what he says there when he has time.

There’s no connection. Migo for a litigant turns his claim into a good claim, but it does not thereby turn him into a witness. This is prevailing through a claim, not through evidence. By contrast, migo for a relative would be a migo that comes to turn him into a witness whose evidence is admissible, and that is something migo cannot do for a relative, because with a relative the problem is admissibility, not credibility.
This has nothing to do with the question of a power of credibility versus “why would I lie.”

I definitely agree with the Rabbi’s answer to his difficulty, but it doesn’t seem to me that this is what he is saying there. There he argues that his whole explanation regarding “this is my son and he is a priest” works only if we understand that migo is a power of credibility and not “why would I lie,” and he says the same regarding a forged document in section 12.

I’d appreciate it if the Rabbi would look there in section 12; he really does connect it to the inquiry about a power of credibility versus “why would I lie.”

I looked again, and that is exactly what he writes there. See, for example, the concluding sentence that distinguishes between an invalid witness and a valid witness who testified falsely.
As for his linking it to the understanding of the law of migo, that is not really necessary. It seems to me correct even if this is a case of “why would I lie.” And perhaps his intent is that if migo were only “why would I lie,” then we would treat it as evidence even if it did not validate his testimony. The proof of migo would stand on its own. But a power of credibility is not evidence; rather, it grants credibility to the claim of the person who is testifying. And in the case of a relative, he does have credibility, except that he is an invalid witness (an admissibility problem).
A side note: throughout his discussion he, like some other later authorities (Acharonim), creates disputes over whether migo is “why would I lie” or a power of credibility, and that is not something that can really be said. It is clear that the rule of “why would I lie” also exists within migo. At most, one can argue about the second rule.

Thank you for the explanation and the response.
Indeed, the approach of the later authorities to completely disqualify the whole notion of “why would I lie” seems rather puzzling, especially since the Kovetz ties it to an amoraic dispute in the passage about a forged document. But in any case, it’s hard to find an approach that explains all the passages on this topic.
Did the Rabbi understand the explanation he offers (in section 12) for the Rashbam’s words in the passage of the forged document? The Rashbam writes (32b, s.v. “Why”): “…And it is not comparable to the other cases of ‘why would he lie’ in the Talmud, where both claims could be true — the one he is making and the one he could have made but did not make — so we say migo. But here it is like ‘why would he lie’ in a place where there are witnesses, for all his force and possession of this land are through this document, so how can we say ‘why would he lie’ regarding the document … after all, he admits that the document is invalid…” And all the medieval authorities (Rishonim) struggled with the Rashbam’s words, because that is how every migo works: in the end, once I make the current claim, it turns out that the underlying claim was false. So the medieval authorities offered rather forced explanations of the Rashbam (Nimukei Yosef, the Rosh), and the Kovetz offers an explanation. I’d appreciate it if the Rabbi could explain it to me.

The distinction is the same distinction that I wrote to you above. Indeed, he has “why would I lie” here too, but he does not have a power of credibility. You want to allow him to prevail by virtue of the witnesses of the forged document, since he could have said that they were genuine — that the document was not forged. But even if he had said that, the victory would not have come through his claim but through witnesses, and now we know that he has no such witnesses. By contrast, in ordinary migo, the victory is through the claim and not through witnesses.
Put differently: the witnesses of a document are invalid by law because of “and not from their written statement.” Normally, however, there is a law of document that turns them into valid witnesses. But when it is known that the document is invalid, then there is no law of document here, and consequently the testimony within it does not exist, so it also cannot give force to the claim that “I had a valid document and it was lost.”

Yes, but even in ordinary migo, where the victory is through the claim, the whole victory is still based on the fact that I had the power to claim “forged.” But after I claimed “I borrowed and repaid,” it turns out that in truth I never could have claimed “forged,” because it became clear that that claim was false. So I never had that power. How is that different from witnesses?

Thank you for the fascinating discussion. I’d be glad if the Rabbi could recommend medieval authorities (Rishonim) and later authorities (Acharonim) who deal with the topic of migo in depth. It’s one of the most interesting topics I’ve encountered in the Talmud.

There is a whole book, Mishpetei HaMigo. There is also a booklet of mine here on the site. You can search for it.

Maybe I’ll sharpen the question: why is it only with claims that we find a “power of claim,” meaning that a claim that could have been true, even though it became clear in the end that it was completely false, gives me the power to prevail with the current claim — whereas a “power of evidence,” meaning evidence that could have been true were it not for my admission and was revealed to be completely false, does not give me the power to prevail with the new claim of “I had a valid document and it was lost”? After all, both the evidence and the claim were ultimately revealed to be completely false. (I’d appreciate it if the Rabbi could expand a bit more; I still haven’t understood.)

I explained it. I always have the ability to make a claim. The fact that I have a claim is not contingent on it being true. A litigant’s claim is not assessed in terms of truth or falsehood, but in terms of his ability to raise it for adjudication. The court will determine whether it is true or false. Therefore, the ability to make another claim is the essence of the mechanism of the power of credibility, and it is not disturbed by the fact that the alternative is false. But a witness or document that does not exist simply does not exist. There you need to actually have a witness or document in order for it to provide a power of credibility.

If a litigant has the ability to raise for adjudication a claim that is a complete lie, then why, in the case of a claim contradicted by witnesses, does he not have the ability to raise it for adjudication? Why shouldn’t he have migo there too? And why is a claim considered something I have the ability to make even if it is false, when a litigant is forbidden to make false claims just as I am forbidden to forge documents, etc.?

He certainly does raise it for adjudication in such a case as well. And in the adjudication they rule against him because of the witnesses.

Yes, but then let him have migo and be believed, just as in a case where there is an admission by the litigant that the claim is not correct, he is still believed despite the litigant’s admission. So now too, even though there are witnesses against him, since he has the ability to claim, he should also be believed.
And why is a claim defined by the ability to make it, rather than by the consideration of whether it is true or not?

Why should he be believed? He has migo, but there are witnesses against him.
I think we’re repeating ourselves. I explained what I could, and the rest — go and learn.

Still, thank you very much, and sorry for going on and on.