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Gemmorah Yevomos 2b:

אלא בתו כיון דאתיא מדרשא חביבא ליה

With regard to his daughter, since this prohibition is actually derived from a homiletical interpretation and is not explicitly stated in the Torah, it is therefore dear to the tanna. Consequently, he listed this novel case of a daughter first.

Rambam writes similarly in Sefer Hamitzvos Negative Commanments 336:

שלא לגלות ערוות הבת; ולמה לא נתפרשה בתורה, מפני שאסר בת הבת שתק מן הבת, ומפי השמועה למדו שאיסור הבת מגופי תורה, כשאר עריות.

Not to have intimate relations with one's daughter. Why was this prohibition not explicitly stated in the Torah? Since the Torah forbade [relations with] one's daughter's daughter, it did not mention [the prohibition against relations with] one's daughter. [Nevertheless, according to the oral tradition, the prohibition against [rela­tions with] one's daughter has the status of a Torah law like the other sexual offenses [and is not considered as Rabbinic in origin].

That's weird because all other similar offenses (Arayos) ARE mentioned in the Torah.

Why would the Torah omit such a basic commandment?

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The Torah mentions explicitly that one's wife's daughter which includes his own daughter (or her daughter from marriage to another man prior to his marriage) is forbidden

(ערוות אישה ובתה, לא תגלה" (ויקרא יח,יז
Rashi explains This Passuk is referring to a daughter born from mother's marriage quoting Gemora Yevamos 97a
(יבמות צז) לא אסר הכתוב אלא ע"י נשואי הראשונה לכך נאמר לא תקח ל' קיחה

The Gemora in Yevamos 3a and the Rambam sefer hamitzvos are referring to a daughter from outside marriage which is not mentioned explicitly in the Torah as explained by Rashi and the Rashba:

בתו עיקר איסורא מדרשא אתיא. כלומר, בתו מאנוסתו דאיירי בה מתניתין דלא נפקא מכלל אשה ובתה וכמו שפרש"י ז"ל

The reason why daughter outside marriage is excluded the Rambam Issurei Bia 2,6 addresses:
If one's Granddaughter from outside marriage is forbidden, this goes without saying that one's daughter must be forbidden as the granddaughter comes from the daughter so since the closeness of relation of the granddaughter makes her prohibited, how much more so the daughter who is even closer relation is prohibited and does not need to be mentioned in the Passuk. This is explained by the Maggid Mishna Issurei Bia 2,6:

בסנהדרין(דף ע') בתו מאנוסתו מנין אמר אביי ק"ו על בת בתו ענוש על בתו לא כ"ש והקשו עליו וכי עונשין מן הדין ותירצו גלויי מילתא בעלמא הוא ורבא יליף לה התם בג"ש. וכתב מצותרבינו ואעפ"י שלא נאמר וכו' ואיסורה מן התורה וכו', הוצרך לזה לפי שיטתו שכתב בספר המצות בעיקר השני שכל הדברים הנדרשים באחת מי"ג מדות אין נקראין ד"ת אלא ד"ס ובכאן ביאר שאין בתו בכלל הזה שהרי ככתובה ממש היא וסמך לו הכתוב על בת בתו שנתבארה בתורה שהיא קרובה לו מחמת בתו ואי אפשר שתהיה בתו מותרת ובתה הבאה מחמתה אסורה
(*Note the reason why the Rambam does not say like Rovo in Sanhedrin 70a (he learns like Abaye) that we learn the daughter is a forbidden relationship from a Gezeira Shava, is because the Rambam writes in his introduction to Sefer Hamitzvos we cannot learn a Ikkar din of the Torah (we can learn punishment just not a shoresh see Kesef mishna Ishus 1,1) from shlosh Esrei Middos.)

The Magggid Mishna seems to say that the Daughter from marriage is mentioned just to show that she is a closer relative than the granddaughter from marriage, the reason why she is forbidden. Abayei and the Rambam derives the Kal Vechomer from there that the daughter from outside marriage is also a closer relative than the grandaughter from outside marriage and is therefore equally forbidden since the Torah has shown that it forbids incest prioritising closeness of relative. (Alternatively the principal מילתא דאתיא בקל וחומר טרח וכתב לה קרא mentioned in Sotah 29a Kiddushin 4b among many other places could apply sometimes the Torah mentions prohibitions that can be worked out from a Kal vachomer and sometimes not.)

  • It appears that your first sentence is totally wrong אשה ובתה means "Her daughter", and explicitly not his. Rahman and the Gemorrah is clear that there's no prohibition from the Torah. That's certain.the question is why, not whether. Please correct. – Al Berko Mar 17 at 8:10
  • @AlBerko her daughter means either her daughter from you or her daughter from someone else this is explicit in Rambam Hilchos Issurei Biah 2,6 and in sefer hamitzvos he is referring to his daughter from outside marriage, read the Maggid Mishna i quoted. – user15464 Mar 17 at 10:11
  • I rewrote the question bringing the Gemmorah - please revise your answer. The question is not on Rambam, no mentioning of Bito is a fact, the question is why. – Al Berko Mar 17 at 14:12
  • @AlBerko i rewrote the answer – user15464 Mar 17 at 15:57
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    @AlBerko i would argue with you that every case of arayos cannot be worked out from a Kal vechomer except this case because there is a precedent of a similar case of granddaughter. If the Torah didn't mention his aunty or sister from his mother or fathers paternal brothers wife i don't think we could have made a Kal Vechomer. in fact i recall that a sister from both mother and father could not be learned from a kal vechomer from a sister just from the mother or just from the father see Maggid mishna i quoted (from its source hebrew books) who talks about this point – user15464 Mar 17 at 17:56

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