If a father says that his son is a mamzer, he gives testimony. He claims that he had relations with a woman who was forbidden to him by a karet (excision). For example, a relative, a close relative. He is not believed. A person is not believed to testify about his close relative. He is considered too close, too involved in the situation. And a karov, a relative, is not allowed to give testimony. And therefore, we do not believe this father.
Even if a husband and wife both show up and say, “The wife is pregnant; she has a fetus in her stomach, and this fetus is a mamzer.” The mamzer: the child was from a different man. The woman committed adultery with another man, and the child they had is in this woman’s stomach now. And that child is therefore a mamzer. They are not believed either. Even though the mother here is certain about the identity of the child’s father, we do not believe her when she says that the child is a mamzer. You need two qualified witnesses, not family members, to give testimony to establish that this child is a mamzer.
According to Rabbi Yehuda, no, we believe them. In both of these cases, the father is believed. The father is believed to say that his child is a mamzer in case number one. We believe the father. And in case number two, obviously, if the mother and the father are coming forward, we believe them as well. We do not believe the mother alone.
The reason, according to Rabbi Yehuda, is that we learn it out from a pasuk (verse): the father is believed to say that his son has a blemish in lineage. We do not learn it out for the mother, only for the father to give the testimony, not for the mother to give that testimony.