Last week we discussed Tosafot Yebamot 47a’s presentation of Rabbeinu Tam’s apparent position that many people are presumptively Jewish, meaning that if they claim to be Jewish, halakhic authorities will believe them without requiring corroborating evidence. According to a beraita on Yebamot 47a, the claim to be a convert is believed only with corroboration or if there is a prior presumption. Rabbeinu Tam commented that this is true only if there is prior knowledge of Gentileness; someone coming literally out of nowhere and claiming to be a convert would be believed.
Rabbeinu Tam (or perhaps Tosafot on his behalf) cited as proof a story from Pesachim 3b, in which a Gentile was given a portion of a Passover sacrifice simply by showing up. He addressed an implicit challenge to his proof: what if that story was not based on presumption, but rather on the probability that most people presenting themselves to eat the sacrifice are Jewish? He responded that most people presenting themselves as Jewish are also Jewish, so Pesachim and Yebamot remain parallel.
However, this response muddies the waters – do we believe the claim to be Jewish because of a presumption, or rather on the basis of probability? We explained last week that presumptions (chazakah), unlike probability claims (rov), can exist even without an evidentiary basis,.
Tosafot cite the beraita on Yebamot 47b we looked at two weeks ago as a second proof for Rabbeinu Tam.
מעשה באחד שבא לפני רבי יהודה,
נתגיירתי ביני לבין עצמי.
א”ל רבי יהודה:
יש לך עדים?
אמר ליה: לאו.
יש לך בנים?
א”ל: נאמן אתה לפסול את עצמך, ואי אתה נאמן לפסול את בניך.
A case: Someone came before Rabbi Yehudah,
and said to him:
“I converted when I was alone”.
Rabbi Yehudah said to him:
“Do you have witnesses?”
He said: “No.”
“Do you have children?”
He said to him:
“You are believed to disqualify yourself, but you are not believed to disqualify your children.”
At first glance this text seems to contradict rather than support Rabbeinu Tam: why don’t we presume the convert to be Jewish? Tosafot, however, start the other way around: why do we presume the children to be Jewish, so that eliminating the father’s testimony leaves their identity legally solid?
ועוד ראיה משמעתין,
דאמר ליה ר”י אי אתה נאמן לפסול את בניך,
There is another proof (for Rabbeinu Tam’s position) from our own sugya,
where R. Yehudah says to him “You are not believed to disqualify your children”,
Why isn’t the father presumed Jewish? Tosafot answer that in fact he is, but a technical mechanism nonetheless prevents him from enjoying all the privileges of Jewish status.
ואיהו גופיה כשר, אלא דשוי נפשיה חתיכה דאיסורא,
אבל אם בא על בת כהן – לא פסלה,
And he himself is also valid, just that ‘he has made himself a slice of prohibition’,
but if he were to have relations with a daughter of a kohen, he does not disqualify her,
as I explained earlier.
The simplest explanation of this mechanism is that it functions in the same manner as an oath.
The upshot of Tosafot is that we presume the father to be Jewish even though he has told us that he is not by claiming to have been invalidly converted. Here the basis for treating the father as Jewish cannot be probability – no one thinks that most people claiming not to be Jewish are actually Jewish. Rather, the basis must be presumption.
The last section of Tosafot notes that a story on Yebamot 45a should not be seen as evidence for Rabbeinu Tam. As background for this story, you need to know that the Talmud records three positions as to the status of matrilineals:
a) that they are mamzerim
b) that they are invalid to marry kohanim
c) that they are no different than Jews with two Jewish parents.
תלמוד בבלי מסכת יבמות דף מה עמוד א
ואף רב יהודה מורה בה להיתירא,
דכי אתא לקמיה דרב יהודה, א”ל: זיל איטמר, או נסיב בת מינך.
וכי אתא לקמיה דרבא, א”ל: או גלי, או נסיב בת מינך.
Rav Yehudah also ruled to permit (a matrilineal Jew to marry a Jew with two Jewish parents),
as when (a matrilineal Jew) came before Rav Yehudah, Rav Yehudah said to him: “Go hide, or else marry a woman like yourself (i.e. matrilineal)”,
and when he came before Rava, Rava said to him: “Either go into exile, or else marry a woman like yourself (i.e. matrilineal)”.
Prima facie, Rav Yehudah and Rava suggest that the matrilineal simply show up in a Jewish community elsewhere, where he will be presumed (in their opinion, correctly) to be a halakhically valid marriage partner for Jews born from two Jewish parents, even where he would be socially ineligible would his heritage be known. This suggests that the new community will not investigate their claim to be Jewish.
Tosafot argue, however, that the new community might have investigated whether he was Jewish, but not have researched his family. Once again, we are left to wonder how he could prove his Jewishness without revealing his parentage. Bottom line, though, this explanation is offered only to reject using the beraita as a proof for Rabbeinu Tam; once Rabbeinu Tam has triumphed anyway, there is no reason to assume the new community checked at all whether he was Jewish.
ומההיא דלעיל (דף מה.) דא”ל זיל גלי אין ראיה,
דשמא לא היו בודקים אלא אם הוא ישראל אם לאו, אבל במשפחתו לא היו בודקין.
From that earlier (narrative” in which he says to him “Go into exile” there is no proof (for Rabbeinu Tam),
as perhaps they would only investigate whether he was Jewish, but they would not investigate his family.
 The text does not actually say that the children are considered Jewish, only that their father’s testimony does not determine their status, but Tosafot presumes that they are considered Jewish.
I would emend based on parallels to “a woman whose halakhic status would change as the result of having sex with a Gentile”
 This is not the right place to discuss the ethics of hiding one’s background from potential spouses, the current social status of matrilineals in the Jewish community, or the relationship of this passage to the issue of mekach taut as a method of freeing agunot.