פירוש על קידושין 4:8

האומר בני זה ממזר הוא אינו נאמן – for he is a relative in relation to him, and a relative is not fit to testify.

Introduction In this mishnah we learn that parents are not believed if they say that their child is a mamzer.

ואפילו שמיהם – the husband and his wife, and it is not necessary that the father alone testifies that he is an illegitimate child, for he is not believed, who is not certain [of the child’s paternity], but even the mother who is certain [of the child’s paternity], she is [also] not believed (see Kiddushin 78b).

If a man declares, “this son of mine is a mamzer,” he is not believed. There are two explanations given for why a father is not believed to say that his child is a mamzer. The first is that the father is related to the child and relatives may not testify. The second is that by saying that the child is a mamzer, the father is testifying against himself and halakhah does not allow self-incrimination.

אפילו על עובר שבמעיה – who does not enjoy the presumption of fitness.

And even if both [the husband and wife] say about the fetus inside her, “it’s a mamzer” they are not believed. Even if both parents say that their child is a mamzer, which means that they are saying that the child is not from the husband but from an adulterous affair, they are not believed.

ר' יהודה אומר נאמנים – The reason of Rabbi Yehuda is that it is written (Deuteronomy 21:17): “[Instead, he must accept [the first-born, the son of the unloved one, and allot to him a double portion of all that he possesses….” – he will be accepted/recognized to others. From here [we learn that] the father is believed to disqualify his son, but the mother is not believed to disqualify her son. And especially regarding his son he believed, but not on the son of his son, for if this son had children, he would not not believed to disqualify them. And the Halakha is according to Rabbi Yehuda.

Rabbi Judah says: they are believed. Rabbi Judah disagrees with both of the previous clauses. In the Talmud Rabbi Judah’s ruling is based on a midrash. That is to say his halakhah is not necessarily logical; rather it is based on his interpretation of Scripture.