Gemara

(Rami bar Chama): Witnesses are not believed to say that they were forced to sign (when we can validate their signatures without them) only when the coercion was monetary. If they say that they signed to save their lives, they are believed.

Objection (Rava): Once they have testified, they cannot retract their testimony!

Suggestion: Perhaps this principle refers only to oral testimony, but not to written testimony.

Rejection: Reish Lakish taught that witnesses signed on a document are treated like testimony that was investigated (and accepted) by Beis Din.

Gitin 36a (Mishnah): It was an enactment for Tikun ha'Olam (the betterment of the world) that witnesses sign a Get.

Question: This is not an enactment, this is mid'Oraisa - "Written in a Sefer and signed"!

Answer #1 (Rabah): The Mishnah is like R. Elazar, who says that Edei Mesirah Karsei (a Get is empowered by the witnesses who saw it given);

Chachamim enacted that witnesses should sign it, for sometimes the witnesses die or go abroad and we cannot validate the Get.

Answer #2 (Rav Yosef): The Mishnah can even be like R. Meir (who says that Edei Chasimah (the witnesses who sign) Karsei. The enactment was that witnesses sign their true names;

(Beraisa): At first, a witness would sign 'I, Ploni, signed for a witness' no matter what his name was. A document could be validated only if the signatures were found on other documents;

R. Gamliel says, they enacted that witnesses sign their true names.

Gitin 3a: Chachamim were stringent to require validation of documents. (Mid'Oraisa it is not needed, due to Reish Lakish's law.)

Yevamos 31b: We didn't enact a date on a Shtar Kidushin since in any case we would not (always) know when she is Chayav Misah. If she keeps the document, she would erase the date. If he keeps the document, he will do the same if he has compassion on her, e.g. his niece;

If we leave it by the witnesses, if they remember the date, they do not need the document. If they do not remember the date, perhaps they will testify based on what they see written. The Torah requires testimony 'mi'Pihem (from their mouths)' not mi'Pi Kesavam (from what they wrote)!

Rishonim

Question (Rif Yevamos 9a): We learned (Kesuvos 28a) that one may testify about the signature of his father, Rebbi or brother. If testimony must be 'mi'Pihem, not mi'Pi Kesavam', one may not testify about his own signature unless he remembers the testimony. All the more so someone else cannot testify about it!

Answer #1 (Rif): The Pesul of 'v'Lo mi'Pi Kesavam' is only when the witnesses still hold the document. They could hide the document, so it is as if they have not testified yet. Therefore, the document is invalid unless they remember the testimony. However, once they signed and gave over the document to a party, they could not retract. It is as if their testimony was investigated in Beis Din, like Reish Lakish taught. Therefore, this is considered mi'Pihem. This is why we can validate documents through the signatures, whether or not the witnesses remember the testimony. "Sados ba'Kesef Yiknu v'Chasuv ba'Sefer v'Chasum v'Ha'ed Edim" connotes that a document is proper testimony, and it is mi'Pihem. In Kesuvos (20a) we learned that one may write in a document testimony that he saw, and testify from it many years later. Rav Huna requires that he remember the testimony by himself. This refers to a document that the witnesses hold.

Question (Nimukei Yosef DH Linsei, and Ra'avad, brought in Sefer Zechus): Here, we are concerned lest the witnesses rely on the document even after the woman received it! Also, according to the Rif, we should have to tear a document if the witnesses forgot the matter before it was given!

Objection (Nesivos ha'Mishpat 46:19): What is the question? It always suffices if witnesses remember being authorized to give document to the lender (even if they did not see the loan). If they do not remember this, surely it is Pasul, like a found document, for perhaps it was never given!

Ra'avad: Rather, any document that could be given immediately is valid, even if it was not given until the witnesses forgot.

Answer #1 (Sefer ha'Zechus): A document that cannot be given to one of the parties (see Yevamos 31b above) is not a Shtar Re'ayah (document of proof)

Objection #1 (Beis Yosef CM 28 DH v'Chosav Nimukei): The Rif does not distinguish thusly! 'Once they signed and gave it over...' implies that we cannot validate any document still by the witnesses unless they remember!

Objection #2 (Nesivos ha'Mishpat, ibid.): The Rif disqualifies when witnesses still hold the document because they could hide it. This applies even if it could be given immediately!

Answer #2 (Nesivos ha'Mishpat): The Rif holds that it is as if Beis Din investigated the testimony only if one who sees the document knows that the loan was given. This applies to Shtarei Hakna'ah, in which the borrower obligates himself immediately, even before the loan is given.

Answer #2 (Ba'al ha'Ma'or): The Pesul is mi'Pi Kesavam, when witnesses decided to write the testimony. Mi'Pi Kesavo is Kosher, i.e. when the borrower, seller or husband commands witnesses to write a document or Get. Verses show that the Torah allows this - "Sados ba'Kesef Yiknu v'Chasuv ba'Sefer v'Chasum v'Ha'ed Edim" and "v'Chosav Lah Sefer Kerisos". However, witnesses may not testify orally about what they saw written. Beis Din must see the document.

Question: Why didn't we enact a date in a Shtar Kidushin, and it with the witnesses? They will take it to Beis Din if needed. Beis Din will ask them if they remembered by themselves. We are not concerned lest witnesses lie and testify based on the document even though they forgot!

Answer (Milchamos Hash-m): The Ba'al ha'Ma'or needs the Rif's answer. Since the date helps only when the witnesses hold it, this is called mi'Pi Kesavam, even though she received it at the time of Kidushin. The Yerushalmi does not permit a woman to remarry based on a document saying 'Ploni died'. She does not need the document, since any woman who says that she heard that her husband died may remarry. Therefore, this is mi'Pi Kesavam.

Answer #3 (Mordechai Kidushin 570, brought in Darchei Moshe CM 28:6): Nevi'im established, according to Hash-m, to accept documents for monetary cases - "v'Chasuv ba'Sefer v'Chasum v'Ha'ed Edim".

Rambam (Hilchos Edus 3:4): The Torah requires testimony to be from the mouth of witnesses, "Al Pi Shnei Edim...", not mi'Pi Kesavam. Mid'Rabanan we decide monetary cases based on documents, even if the witnesses are not alive, lest people be discouraged from lending. We do not rule about fines based on documents, and all the more so not about lashes or Galus. Those must be mi'Pihem v'Lo mi'Pi Kesavam.

Source (Magid Mishnah): Chachamim enacted to sign documents (Gitin 36a).

Question (Lechem Mishneh): It says there that witnesses sign on documents mid'Oraisa! (The enactment requires signing, or to sign one's actual name.)

Kesef Mishneh: This is an alternative answer to the Rif's question. The enactment is for monetary matters. It cannot apply to a Shtar Kidushin to obligate Misah. The Rashbatz asks, "Sados ba'Kesef Yiknu" proves that documents are mid'Oraisa! This is not difficult, for that verse is from Nevi'im.

Question (Kesef Mishneh): In many places (e.g. Kesuvos 28a) we say that the need to validate documents is only mid'Rabanan, but mid'Oraisa it is as if Beis Din investigated the testimony!

Answer (Lechem Mishneh): The Tur (CM 28) says that the Rambam holds like Rashi, that mid'Oraisa testimony must be oral. The Beis Yosef (DH v'Da) says that the Tur's text of the Rambam below (9:11) did not mention 'or by one who could testify orally', which is like R. Tam. The Beis Yosef concurs, to avoid a contradiction in the Rambam. I support our text; the Rambam holds like R. Tam. Mid'Oraisa, a document works if the witnesses are alive and remember the testimony, or send it to Beis Din. The Rambam says that mid'Rabanan we decide monetary cases based on documents, i.e. even if the witnesses died.

Question (Ramach): Signed documents work also for non-monetary cases, e.g. Gitin, even according to R. Elazar who says that Edei Mesirah Karsei!

Poskim

Shulchan Aruch (28:12): We decide monetary cases based on documents, even if the witnesses are not alive, lest people be discouraged from lending.

SMA (43): It must be a proper document. This was an enactment for Tikun ha'Olam.

Rebuttal (Shach 14): The Shulchan Aruch and SMA are based on the Rambam. However, Chachamim of all generations disagrees. The Ramban (Hasagos on Sefer ha'Mitzvos) refutes the Rambam. Megilas Esther's defense is inadequate. In many places the Gemara says that documents work mid'Oraisa. Witnesses signed on a document are like testimony that was investigated; validation is a stringency mid'Rabanan (Gitin 3a).

Defense (Taz): There is a contradiction. It says "Al Pi Shnei Edim", and it says "v'Chasuv ba'Sefer v'Chasum". The Rambam concluded that documents work mid'Oraisa only where verses explicitly say so, e.g. Gitin. For such documents validation is only mid'Rabanan, and all the more so for document that themselves are only mid'Rabanan!

Aruch ha'Shulchan (17): All agree that documents that make acquisitions are mid'Oraisa. The argument is only about documents that are a mere proof.