1) TESTIMONY VIA WRITTEN AFFIDAVIT
QUESTION: The Gemara states that a witness may record in writing his testimony about an event and refer to his written record to remind himself of the details of the event when he testifies in Beis Din even many years later. The Gemara quotes Rebbi Yochanan who says that this form of testimony is valid even when the witness would not remember the details of the event without reading his record of them.
TOSFOS (DH v'Rebbi Yochanan Omer) explains that Rebbi Yochanan does not mean that a witness who has no recollection at all of the event may testify based on what he wrote. The Gemara in Yevamos (31b) clearly states that such testimony is not acceptable. Rather, Rebbi Yochanan means that when the witness has a general recollection of what happened he may augment his testimony with details based on his own written record from the past.
The Gemara's ruling that a witness may give testimony based on what he wrote earlier raises a question. Why does the witness' note differ from a common Shtar? A common Shtar records the details of a transaction, and those details recorded in the Shtar are validated by the signature of witnesses. The validated Shtar may be used as testimony in Beis Din as the Gemara states (18b), "witnesses whose signatures are signed on a Shtar are considered as though their testimony has been [presented to and] examined by Beis Din." Why is a private note written by a witness not considered like testimony given in Beis Din?
ANSWER: TOSFOS asserts that in order for a Shtar to be valid it must be written with the "Da'as ha'Mischayev" -- "the consent of the one who is being obligated [by the words of the Shtar]." Both parties involved in the transaction must agree to write the specific details in the Shtar. In contrast, the memoirs of a witness are written without the consent of the parties involved, and thus his written record does not attain the status of a Shtar.
However, this approach needs clarification. The testimony that a witness delivers does not depend on what the parties involved want the witness to see, hear, or report. Why does a witness' written record of the event not attain the status of a Shtar simply because it was not written with the "Da'as ha'Mischayev"?
RAV ISER ZALMAN MELTZER in EVEN HA'AZEL (Hilchos Edus 8:1) explains that the reason why a Shtar is a valid attestation of the transaction is that the signatures of the witnesses constitute an affirmation of the details at the time they sign. Their signatures are like a formal notarization; it as if they are testifying that whatever is written in the document is true. Accordingly, the testimony of the witnesses is actually finished at the moment they sign the Shtar. At that moment the details about which they are testifying are clear and fresh in their minds. Later, if they are summoned to Beis Din to testify and they do not remember the details of the event, the Shtar is not considered invalid because their testimony is considered to be the same as it was at the time they signed the Shtar. This is what the Gemara earlier (18b) means when it says, "witnesses whose signatures are signed on a Shtar are considered as though their testimony has been [presented to and] examined by Beis Din." At the time they sign the Shtar, it is as if Beis Din was present and accepted their testimony.
Why, though, does Beis Din accept all of the details written in a Shtar as true, as if the testimony has already been examined and scrutinized? It must be because the Shtar was written with the "Da'as ha'Mischayev." The "Mischayev" (the borrower) certainly would not let anything untrue be written in the Shtar to his detriment. Accordingly, the details written in the Shtar with the "Da'as ha'Mischayev" are treated as validated testimony. In contrast, a record of the details which was written without the "Da'as ha'Mischayev" is not treated as testimony examined by Beis Din. Any form of testimony in writing, such as the written memoirs of a witness, or a written affidavit the witnesses send to Beis Din, does not have the credibility of testimony which has already been examined by Beis Din.

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