1) ACCEPTING THE TESTIMONY OF A RELATIVE
QUESTION: The Gemara discusses a case in which a person claims, "This is my son, and he is a Kohen." Rebbi says that he is believed inasmuch as we may give the son Terumah to eat, but we may not permit him to marry the daughter of a Kohen Meyuchas. Rebbi Chiya says that if we accept his testimony with regard to Terumah, we must also accept his testimony with regard to Yichus, and if we do not accept his testimony with regard to Yichus, we should not accept it at all. (The Gemara understands that Rebbi Chiya maintains that the testimony is not accepted at all.)
Why is a father not believed to testify that his son is a Kohen? The issue of Kehunah is a matter of Isur, for which a single witness is believed ("Ed Echad Ne'eman b'Isurim").
ANSWER: The ROSH in Gitin (5:12) writes that a person is not believed to testify about himself that he is a Kohen in order to be permitted to eat Terumah, because accepting his claim that he is a Kohen would provide him and his descendants with a great benefit, and therefore a person who makes such a claim is suspected of lying. The Rosh apparently understands that the only reason why one is not believed to testify that he is a Kohen is the concern that he is lying for personal gain. This may also be the reason why a father is not believed to say that his son is a Kohen.
TOSFOS here points out that the statement, "This is my son, and he is a Kohen," implies that there are two details which are unknown and which the father is revealing: the first is that the child is his son, and the second is that the child is a valid Kohen. Tosfos asks that if it is not known that the child is his son, why is the father not believed when he says that his son is a valid Kohen with a "Migu" that he could have said that the child is not his son and is a Kohen? Had he made that claim, he would have been believed because he would have no ulterior motive, as he would not be claiming to be the child's father. He would be testifying about an unrelated person for whom he is believed to testify.
Tosfos answers that he is not believed even though he has a "Migu" because, according to his present statement, he is a relative and one is not believed to say that his relative is a Kohen.
This logic seems flawed. According to the Rosh, the only reason why a relative is not believed for Kehunah is because he might be lying. If there is a "Migu," however, that "Migu" provides reason to believe that he is not lying. Why, then, is he not believed even though he is a relative of the one for whom he testifies? Apparently, Tosfos understands that there is some other reason for why a relative is not believed. (The TOSFOS HA'ROSH rejects Tosfos' logic and suggests another answer to Tosfos' question. See KOVETZ HE'OROS #65, who suggests a Chidush with regard to the Pesul of a Karov (relative) who testifies, which may be applied here.)
Perhaps the answer is that although with regard to Kehunah a relative is not accepted as a witness because he is suspected of lying, there is a different reason for why his testimony is not accepted even when there is no suspicion that he is lying (such as when he has a "Migu"). Since a relative is not valid as a witness when two witnesses are necessary even though he is not suspected of lying, the Chachamim made a general decree that a relative is not accepted as a witness for Kehunah just as he is not accepted for normal, two-witness testimony. Therefore, even in a case where there is a "Migu" (and thus there is reason to assume that he is not lying), the testimony of a relative is not accepted. (E. KORNFELD)