(Beraisa): A person may write in a document testimony (that he saw), and (orally) testify from it many years later.

Opinion #1 (Rav Huna): He can testify only if he remembers the testimony by himself.

Opinion #2 (R. Yochanan): He can testify even if he does not remember it by himself.

(Rabah): We learn from R. Yochanan that if two witnesses saw testimony and one forgot it, the other may remind him.

Question: May a party in the case remind a witness of the testimony?

Answer #1 (R. Chaviva): Yes.

Answer #2 (Mar bar Rav Ashi): No.

The Halachah is, he may not, unless the witness is a Chacham.

Rav Ashi knew testimony for Rav Kahana, but forgot it, even after Rav Kahana tried reminding him of it. Later, he remembered and testified for him. Rav Kahana was taken aback.

Rav Ashi: Don't worry. I do not rely on what you said. I put my mind to it and remembered.

20b (Mishnah): Old far mounds are Tamei, and new ones are Tahor;

R. Meir says, sixty years old is considered old;

R. Yehudah says, if no one remembers before it was built, it is old.

(Rav Chisda): We learn from R. Meir that a person remembers testimony for 60 years, but no longer.

Rejection: It is not incumbent on a person to remember Tum'ah in a mound. He has a responsibility to remember testimony, so he (puts it to heart and) remembers it longer.

Rishonim

Rif and Rosh (Kesuvos 7b and 2:16): Rav Hai Gaon says that the Heter to write testimony in a document refers to a witness who saw testimony, and much later the lender asked him to write a document or testify in Beis Din. This is permitted, even after many years. He need not be concerned that the loan was paid or pardoned. Rav Huna requires the witness to remember the testimony by himself. R. Yochanan allows even if he remembers after others remind him. Rava learns from R. Yochanan that if two witnesses saw testimony and one forgot it, the other may remind him. Some say that the Gemara does not discuss a document given to the lender, rather, a Sefer of recollections that one keeps in his house. A party asked him to testify, and he finds in his Sefer the details of the testimony. He may testify even after many years. Rav Huna requires the witness to remember the testimony by himself; R. Yochanan allows even if he remembers after seeing it written. From R. Yochanan, Rava infers that if one witness forgot testimony, the other may remind him. According to the first Perush, this is included in R. Yochanan's words. What does Rava add?! Therefore, the latter Perush is better.

Ran (DH Kosev): Also according to the first Perush, Rava adds a Chidush! One might have thought that others can remind a witness (Reuven), for he would not rely on them if he did not remember, but the other witness may not remind Reuven, lest Reuven rely on him even though Reuven himself does not remember. However, the Tosefta favors the Rif's Perush.

Rosh: It seems that in any case one may rely on Rav Hai Gaon's Perush to write testimony even many years later without concern that the loan was paid or pardoned. Had the borrower paid it, he would have told the witnesses so they will not write or testify about the loan. If he did not do so, he caused himself a loss.

Nimukei Yosef (Yevamos 9a DH Linsei): "Al Pi Shnei Edim" teaches that witnesses may not testify from what they wrote (unless they remember it), and also that one may not write his testimony and send it to Beis Din.

Rosh (ibid.): One who remembers testimony can always testify about it. He need not be concerned that perhaps it has been too long and he does not remember it well. As long as he puts it to mind and remembers, he may testify.

Rambam (Hilchos Edus 8:5): Whether one writes testimony in a document, or it is found in his ledger in his handwriting 'Ploni made me a witness on this day about this matter...', he may testify only it he remembers the testimony by himself or when reminded of it. If not, this is Ed mi'Pi Ed. It is like one (David) who heard from a reliable source that Reuven owes Shimon a certain amount, and David testified even though he does not know.

Rebuttal (Ra'avad): The first law is wrong. (Rather, one may testify about what he wrote in a document even if he does not remember it.)

Radvaz and Migdal Oz: The Ra'avad rules like R. Yochanan because Rava derives another law from R. Yochanan.

Kesef Mishneh: The Gemara permits to testify based on one's own written testimony only if he remembers it. The Rambam explains that this is whether he wrote in his ledger or in a document. The Gemara supports this; it says that one may write in a document. The case is, the document cannot be validated without the witness. The same applies to testimony in one's ledger.

Rosh (Yevamos 3:7): If we would leave a Shtar Kidushin with witnesses, if they would testify that they signed it, this is mi'Pihem. The concern is lest they testify about what happened as if they remember, but really they do not; they only see what they signed. This is mi'Pi Kesavam. R. Tam would instruct witnesses far from Beis Din to write their testimony and send it to Beis Din. This is mi'Pihem because they remember it. However, Rashi (Devarim 19:15) says that "Al Pi Shnei Edim" disallows this.

Question (Beis Yosef CM 28 DH Kosav Rashi): If a person wrote testimony that he saw, he may testify about it later only if he remembers the testimony (Kesuvos 20a). According to R. Tam, even if he does not remember, Beis Din may rely on what he wrote!

Answer (Beis Yosef): The case is, the witness does not have in Beis Din what he wrote.

Poskim

Shulchan Aruch (28:13): As long as one remembers he may testify. He need not be concerned that since it has been a long time he does not remember well.

Beis Yosef (DH v'Chol): The Rosh learns from Kesuvos 20b, which allows one to testify for 60 years about Tum'ah of mounds, but even longer for things that are incumbent on him to remember. Rashi says that the Halachah is different when one was asked to be a witness. This implies that if he was not made a witness, the law is like regarding mounds. This is reasonable, but I did not find anyone else who says so. Perhaps this is because the Halachah follows R. Yehudah against R. Meir, and R. Yehudah permits in any case as long as he remembers.

SMA (44): Even if one did not intend to be a witness at the time, he may testify about it in the future as long as he remembers.

Shulchan Aruch (13): Even if a witness forgot testimony and remembers it only after seeing what he wrote in his ledger, and then he remembers the testimony, he may testify.

Beis Yosef (DH u'Mah she'Chosav v'Afilu): Even though most disagree with Rav Hai Gaon's Perush, the law is true.

Shulchan Aruch (14): Similarly, if he remembers after someone reminds him, he may testify, even if the other witness reminded him. However, if a party reminded him he may not. However, if the witness is a Chacham even this is permitted, for surely he would not testify if he did not remember.

Aruch ha'Shulchan (18): A witness may say 'this I remember and these matters I forgot'. He may even say 'I found this in my ledger, but I do not remember it'. Beis Din will know not to rely on it. Even relatives of a Ba'al Din may remind a witness of testimony, but the Ba'al Din himself may not. A Ba'al Din may tell Beis Din or a Shali'ach to remind a witness about testimony. Since the witness does not hear from the Ba'al Din himself, we are not concerned.