One of the subjects our Gemara addresses is how we can establish a person’s lineage. How can we be certain that someone is a kohen, for example? If he regularly eats teruma, does that prove that he is a kohen? If he does nesi’ut kapayim – blesses the people with birkat kohanim – will that prove that he is a kohen? And what if we have a shtar – a document signed by witnesses – in which an individual is referred to as a kohen? Will that document be accepted as proof that he is a kohen?
According to the Gemara, if we have a document, signed by witnesses, which says “So-and-so the kohen borrowed a sum of money from a certain person” we find a disagreement between Rav Huna and Rav Hisda on the question of whether witnesses focus on the main point of the shtar – i.e. the sum of the loan – or they are attesting to the veracity of the entire document, which includes the statement that the borrower is a kohen.
At least part of the question here is dependent on a more basic issue. Does a Jewish court accept written testimony?
In general, the Gemara follows a rule which states, “Mi-pihem ve-lo mi-pi ketavam – the halakha accepts verbal testimony, but not written testimony.” On the other hand, the Gemara has a principle that, with regard to documentation of loans, purchases, and the like, we say, “Eidim ha-hatumim al ha-shtar na’aseh ke-mi she-nehkerah eidutam be-beit din – witnesses who have signed a legal document are considered to have had their testimony authenticated in the courtroom.” Nevertheless, there is room to distinguish between the central issue for which the shtar was written, where we would say eidim ha-hatumim al ha-shtar na’aseh ke-mi she-nehkerah eidutam be-beit din, and incidental points mentioned in the shtar, where we would apply the rule mi-pihem ve-lo mi-pi ketavam and would rule that they cannot be viewed as reliable testimony.
In the end, the Gemara distinguishes between such priestly benefits as eating teruma in modern times on a Rabbinic level, where such testimony would be sufficient, and establishing him as a member of the family of kohanim on a Torah level where it would not be (see the Rambam’s Mishneh Torah, Hilkhot Issurei Bi’ah 20:9).