In this lesson we will study Mishnah 8 in Chapter 4 of Maseches Kiddushin, which deals with the question of a person's credibility to determine the halachic status of his own son.
"Ha'omer beni zeh mamzer - eino ne'eman": - one who says, "This son of mine is a mamzer," is not believed.
A person who points to his son and declares that he is a mamzer, meaning that he was born from a forbidden relationship, is not believed, even though he insists that this is truly the case.
Two reasons are given for why he is not believed:
Relationship: A father is a relative of his son, and a relative is disqualified from testifying.
A person cannot render himself wicked: A person is not believed regarding a matter that would render him wicked, that is, a transgressor of the Torah. For in order for his son to be a mamzer, he would have had to violate the prohibitions of forbidden relations.
A precise point is worth noting: the second reason applies specifically when the father claims that the son came from a forbidden relationship of his own. If he claims that his wife had relations with another man, his words do not involve rendering himself wicked. However, the first reason, his relationship to the son, still stands in that case as well.
"Va'afilu shneihem omrim al ha'ubar shebime'eha mamzer hu": - and even if both of them say about the fetus in her womb that he is a mamzer.
Even if the husband and the wife together testify about the fetus presently in her womb that he is a mamzer, and she says that he indeed came from relations with another man, they are not believed. It should be noted that seemingly there would be room to accept their words from two angles: the woman knows with certainty whether she had relations with another man, and she may even know that she did not have relations with her husband at that time; and in addition, a fetus has no chezkas kashrus, since he is only beginning his existence and has no established record of valid status. Nevertheless, the Mishnah rules "einam ne'emanim" - they are not believed, since once again we return to the reason of their relationship to him.
The opinion of Rabbi Yehudah:
Rabbi Yehudah disagrees and says: "ne'emanim" - they are believed. And not only that, but even the father alone, without the mother, is believed to say about his son that he is a mamzer. Rabbi Yehudah's reasoning is based on the verse concerning the firstborn son - "yakir" - he shall recognize: the father has the power to recognize his son, that is, to identify him before others and to establish who his son is. From here Rabbi Yehudah learns that the father is granted credibility to determine and to inform us of the halachic status of his son.
The limit of this credibility:
This credibility applies to the son alone, and not to the grandson. That is, if his son already has children, and his statement about the son would affect them as well, he is no longer believed even with regard to his son himself.
In summary: The Mishnah rules that one who says about his son that he is a mamzer is not believed, both because of his relationship to him and because of the principle that a person cannot render himself wicked. Even both parents together are not believed about the fetus in her womb, despite the fact that it has no chezkas kashrus. Rabbi Yehudah disagrees, and on the strength of the verse "yakir" he grants the father credibility to determine his son's status, a credibility limited to the son alone, and not where his words affect his grandchildren as well.