Is Logic Enough to Prohibit Father-Daughter Incest?
Although the Torah’s list of incestuous relationships in Leviticus 18 and 20 might feel exhaustive, neither text contains a prohibition against father-daughter incest, a jarring omission. In her, “Does the Torah Prohibit Father–Daughter Incest?” (TheTorah 2019), Eve Levavi Feinstein notes that among contemporary critical scholars, some explain the gap as proof that no such prohibition existed in ancient Israel, while others (including Feinstein herself) see its absence as a result of “some mishap” or an “accidental omission.” Neither approach would have been acceptable to traditional readers, who searched for an authoritative source for this prohibition.
The Prohibition against daughters: An A Fortiori Argument?
A first source for the prohibition against father-daughter incest was found in a related prohibition against incest with a granddaughter:
ויקרא יח:י עֶרְוַת בַּת בִּנְךָ אוֹ בַת בִּתְּךָ לֹא תְגַלֶּה עֶרְוָתָן כִּי עֶרְוָתְךָ הֵנָּה.
Lev 18:10 The nakedness of your son’s daughter, or of your daughter’s daughter—do not uncover their nakedness; for their nakedness is yours.
Rav (3rd cent. Babylonia), cited in the Jerusalem Talmud, uses a fortiori reasoning from this verse to prove that father-daughter incest is also prohibited:
ירושלמי יבמות א:א [יא::] רב אמר: "אם על בת בתו הוא מוזהר לא כל שכן על בתו?! אם על בת בתו הוא ענוש לא כל שכן על בתו?!"
j. Yebamot 1:1 [11d] Rav said: “If one is prohibited to have relations with his daughter’s daughter (Lev 18:10), is his daughter not prohibited all the more so?! If one is punished for having relations with one’s daughter’s daughter, for one’s daughter not all the more so?!”
Eliyahu Rabbah: Making Use of Simple Arguments
The early-geonic period Seder Eliyahu Rabbah, from the latter third of the first millennium C.E., is a work that expounds rabbinic law to nonrabbinic (or less rabbinicized) Jews, that is, Jews who were less familiar with or less committed to rabbinic Judaism. One series of passages has an interlocutor inquire of Elijah (the imagined narrator) about the source for various prohibitions. This section begins with Elijah meeting people who seem to endorse Karaite-like scripturalism, since they are knowledgeable about biblical texts but not about rabbinic ones:
אליהו רבה טו פעם אחת הייתי עובר ממקום למקום, מצאתי אדם אחד שיש בו מקרא ואין בו משנה... טז בא חבירו וישב כנגדו, אף בזה יש בו מקרא ואין בו משנה...
Eliyahu Rabbah 15 Once I (Elijah) was travelling from place to place, and I found a man who was learned in Bible but not in Mishnah… 16 [Then] his friend came and sat opposite him, and he too was learned in Bible but not Mishnah…
This second person asks about the prohibition against father-daughter incest, since it is absent from the Torah. In response, the text offers Rav’s a fortiori reasoning, as found in the Jerusalem Talmud:
אליהו רבה טז אמר לי: "רבי בת חמורה או בת בת?" אמרתי לו: "בני, בת ערוה ובת הבת ערוה. כך הבת כמוה בת הבת."
Eliyahu Rabbah 16 He said: “My master, which is worse, [intercourse with] a daughter or a daughter’s daughter?” I said to him: “My son, a daughter is incest and a daughter’s daughter is incest. A daughter is just as forbidden as a daughter’s daughter.”
אמר לי: "רבי, והלא אין כתוב בתורה 'ערות בתך לא תגלה'?" אמרתי לו: "בני ולאו קל וחומר הוא? ומה אם בת בנו ובת בתו של אדם אסורה לו, בתו לא כל שכן?!"
He said to me: “My master, but the Torah never writes, ‘do not reveal the nakedness of your daughter.’” I said to him: “My son, is it not a logical a fortiori argument? If a son’s daughter and a daughter’s daughter are forbidden, his own daughter all the more so!”
The a fortiori argument soon also became the preferred approach among Karaite scholars.
Evidence that Logical Arguments are Legitimate: The Karaite Approach
The earliest non-rabbinic writing about the prohibition of incest with one’s daughter is ascribed to ‘Anan ben David (ca. 715–ca. 795), who was once thought of as the “founder” of Karaism but is now understood to be a forerunner of this ninth- or tenth-century movement. ‘Anan is said to have used the Torah’s silence about father-daughter incest in order to support the validity of a fortiori reasoning in general.
Similarly, in his critical evaluation of Rabbi Ishmael’s thirteen middot, the Iraqi Karaite sage, Yaʿqūb al-Qirqisānī (ca. 890–ca. 960) includes the prohibition against father-daughter incest under the heading of a fortiori arguments. Discussing the Levitical prohibitions, he writes: “if the granddaughter is prohibited, the daughter—who is more closely related—should be prohibited all the more so.”
In Byzantium, leading Karaites Judah Hadassi (twelfth century) and Elijah Bashyatchi (fifteenth century) used a fortiori reasoning to derive a prohibition against father-daughter incest. In the introduction to the latter’s Adderet Eliyahu, the author offered a lengthy disquisition on scriptural interpretation. In the fourth topic, a fortiori reasoning, he explained:
כי התורה לפעמים אסרה הדבר הקל ולא אסרה החמור ואסורו מהקש קל וחומר כגון שאמר הכתוב ערות בת בנך או בת בתך לא תגלה. והנה לא הזכיר אסור הבת וידענו אסורה מקל וחומר שאם בת הבת אסורה כל שכן הבת עצמה.
Sometimes, the Torah forbade a lenient matter but did not forbid the stringent matter. Its prohibition is from the a fortiori form of reasoning. For example, scripture said “do not reveal the nakedness of your son’s daughter or your daughter’s daughter” (Leviticus 18:10). Now, the prohibition against the daughter is unmentioned. We know its prohibition from an a fortiori argument: if the daughter’s daughter is forbidden, all the more so the daughter herself.
While this approach appears here and there in rabbinic literature, for the most part, the rabbis dismissed the a fortiori argument as a basis for the prohibition and looked to a different biblical proof for the prohibition of father-daughter incest.
A Subcategory of “A Woman and Her Daughter”: The Mishnah
The Mishnah lists two categories of violations that come with the penalty of burning:
משנה סנהדרין ט:א וְאֵלּוּ הֵן הַנִּשְׂרָפִין: הַבָּא עַל אִשָּׁה וּבִתָּהּ, וּבַת כֹּהֵן. וְיֵשׁ בִּכְלַל אִשָּׁה וּבִתָּהּ: בִּתּוֹ, וּבַת בִּתּוֹ, וּבַת בְּנוֹ, וּבַת אִשְׁתּוֹ, וּבַת בִּתָּהּ, וּבַת בְּנָהּ, חֲמוֹתוֹ, וְאֵם חֲמוֹתוֹ, וְאֵם חָמִיו.
m. Sanhedrin 9:1 And these are the ones whose sins are punished by burning: one who has sexual intercourse with a woman and her daughter, and the daughter of a priest [who prostituted herself]. In the category of “a woman and her daughter” are: his daughter; the daughter of his daughter; the daughter of his son; the daughter of his wife; the daughter of her daughter; the daughter of her son; his mother-in-law; the mother of his mother-in-law; and the mother of his father-in-law.
The punishment for the daughter of a priest who prostituted herself comes from Leviticus 21:9. The punishment for marrying a wife’s relatives comes from the confluence of two verses. First, the collection of prohibitions in Leviticus 18 includes a prohibition to marry “a woman and her daughter”:
ויקרא יח:יז עֶרְוַת אִשָּׁה וּבִתָּהּ לֹא תְגַלֵּה אֶת בַּת בְּנָהּ וְאֶת בַּת בִּתָּהּ לֹא תִקַּח לְגַלּוֹת עֶרְוָתָהּ שַׁאֲרָה הֵנָּה זִמָּה הִוא.
Lev 18:17 Do not uncover the nakedness of a woman and her daughter; nor shall you take [into your household as a wife] her son’s daughter or her daughter’s daughter and uncover her nakedness: they are kindred; it is depravity.
While this verse says nothing about burning, a similar one in Leviticus 20 that addresses “a woman and her mother” does:
ויקרא כ:יד וְאִישׁ אֲשֶׁר יִקַּח אֶת אִשָּׁה וְאֶת אִמָּהּ זִמָּה הִוא בָּאֵשׁ יִשְׂרְפוּ אֹתוֹ וְאֶתְהֶן וְלֹא תִהְיֶה זִמָּה בְּתוֹכְכֶם.
Lev 20:14 If a man takes a woman and her mother [as wives], it is depravity; both he and they shall be put to the fire, that there be no depravity among you. 
The Mishnah seems to understand these verses as mirror-image articulations of the same underlying violation. This violation is relations with direct relatives of one’s wife. Although the Mishnah does not articulate its prooftext, the Talmud makes this connection explicit, using a gezeirah shavvah, a rabbinic principle that the appearance of the same word in two contexts means that one can learn from one to the other, in this case, the word זִמָּה, “depravity”:
בבלי סנהדרין עה. תנו רבנן: "איש אשר יקח את אשה ואת אמה"—אין לי אלא אשה ואמה, בתה ובת בתה ובת בנה מניין? נאמר כאן "זימה" ונאמר להלן "זימה", מה להלן בתה ובת בתה ובת בנה, אף כאן בתה ובת בתה ובת בנה.
b. Sanhedrin 75a Our rabbis taught: “A man who takes a woman and her mother” (Lev 20:14)—I only have [the case of] a woman and her mother, from where do we know [that he receives the same punishment for] her daughter, her daughter’s daughter, and her son’s daughter? Here it says “depravity” (Lev 20:14) and there is says “depravity” (Lev 18:17). Just as there it refers to the daughter, the daughter’s daughter and the son’s daughter, so too here [it includes] the daughter, the daughter’s daughter and the son’s daughter.
The Mishnah, as usual, does not explain how it decided to include the prohibition of incest between father and daughter in the category of “a woman and her daughter.” On this score, the Talmud notes a problem that derives from reading Leviticus 18:17 (woman and her daughter) and Leviticus 20:14 (woman and her mother) together, namely, the use of the word ל.ק.ח, “to take” in both.
The Illegitimate-Daughter Gap
The problem is that verb ל.ק.ח generally refers to a man taking a woman as a wife, that is, through marriage (see, e.g., Deuteronomy 24:1). Thus, these verses that address the daughter or mother of the woman, and by extension, her granddaughters and grandmothers, are only forbidden if the man married the first woman.
What is the law about daughters/mothers of a woman he had relations with but did not marry? According to the rabbis, the mother/daughter of a woman whom the man has not married is not prohibited:
ירושלמי סנהדרין ט:א [כו::] כתיב "ערות אשה ובתה לא תגלה" וכתיב "ואיש אשר יקח את אשה ואת אמה זמה היא" בכולהם כתיב שכיבה ובה כתיב לקיחה ללמדך שאינו חייב על השנייה עד שתהא לקוחה לו.
j. Sanhedrin 9:1 [26d] It says (Lev 18:17) “Do not uncover the nakedness of a woman and her daughter.” And it says (Lev 20:14) “If a man takes a woman and her mother, it is depravity.” In all the verses [in this section] it uses the verb “lie with” but here it uses “take.” This teaches you that the man has not made himself liable to punishment for [lying with] the second woman unless he has married the first woman.
This left the rabbis with yet another problem: the daughters of a man’s wife are prohibited (Leviticus 18:17) and biological granddaughters are prohibited, even if they are not the product of a marriage (as per Leviticus 18:10, which does not use the word “to take”,) but what of daughters born out of wedlock?
We Don’t Punish Based on A Fortiori Reasoning
The Babylonian Talmud raises the possibility discussed above (i.e., Rav’s view in the Jerusalem Talmud), that the prohibition against father-daughter incest for daughters born out of wedlock can be proven through a fortiori reasoning:
בבלי סנהדרין עו. בתו מאנוסתו מנין? האמר אביי: "קל וחומר, על בת בתו ענוש על בתו לא כל שכן?!"
b. Sanhedrin 76a What is the source for the prohibition against incest with a daughter born outside of wedlock? Didn’t Abayye say: “An a fortiori argument: if one is punished for relations with his granddaughter, how much more so for relations with his daughter?!”
And, yet, the Babylonian Talmud quickly rejects this option, since it violates a generally accepted legal principle:
בבלי סנהדרין עו. וכי עונשין מן הדין? גלויי מילתא בעלמא הוא.
b. Sanhedrin 76a But do we really administer punishments based on logical arguments? This [logical inference] merely discloses a matter [of law].
A Double Gezeirah Shavvah
Thus, in several passages, the Babylonian Talmud turns to yet another gezeirah shavvah, this time using the word henna (“they are”):
בבלי כריתות ה. אמר אביי "לעולם אל תהי ג"ש קלה בעיניך שהרי בתו מאנוסתו הן הן גופי תורה ולא לימדה הכתוב אלא מג"ש" דאמר רבא "אמר לי ר' יצחק בר אבדימי 'אתיא הנה הנה [לאיסורא] אתיא זמה זמה [לשרפה]'".
b. Keritot 5a Abayye said: Never treat a gezarah shavvah lightly, since a daughter born out of wedlock is the “essence of the Torah” but is only derived from a gezeirah shavvah. As Rava said, “Rabbi Isaac bar Avdimi said to me: ‘This prohibition is derived from the words henna (“they are”) and henna (Leviticus 18:10, 17), teaching the prohibition, and zimmah (“depravity”) and zimmah (Leviticus 18:10, 20:14), teaching that the punishment is being burnt to death.’”
The additional gezeirah shavah connects the phrase כִּי עֶרְוָתְךָ הֵנָּה “for they are your nakedness” in the verse about the man’s own granddaughters (Lev 18:10)—which the rabbis interpret as granddaughters born from a daughter born out of wedlock—with the phrase שַׁאֲרָה הֵנָּה “they are her kindred” (Lev 18:17) in the verse about daughters and granddaughters (from a wife). Just as the latter is prohibited so, too, is the former.
Saadia and the Anti-Karaite Polemic
The complex approach of the Babylonian Talmud is endorsed by Rav Saadia ben Joseph Gaon (882–942), who polemicized against Karaites more than any other Rabbanite. In his Judeo-Arabic explanation of the thirteen middot of R. Ishmael, part of his commentary on Leviticus, Saadia writes:
There are matters that are not explicit in scripture but tradition equated them. … As it says, prohibiting the daughter of his wife, “do not reveal the nakedness of a woman and her daughter” (Leviticus 18:17), but your daughter is not mentioned. The tradition arrived, adding a daughter to the prohibition of his granddaughter, just as it added the daughter of his wife to her granddaughter. The word henna stated here is like the word henna stated there.
Saadia’s recapitulation of the rabbinic gezeirah shavvah emphasizes that this teaching is a received tradition, not a law created by the rabbis. But did he choose the gezeirah shavvah over a fortiori reasoning only because the Talmud rejects the latter, or is there more to it than that? The Karaite-Rabbanite debate over the use of human reasoning in the development of new law in general, and in the realm of incest law in particular, lends greater context to Saadia’s statement here, especially because several Karaites ascribe to Saadia a general rule not to compare prohibited relationships in order to derive new rulings from them. Such an ascription is quite plausible.
Indeed, in his Book of the Commandments, poetry, and elsewhere, Saadia even insisted that the secondary class of prohibited relationships (known in rabbinic literature as the sheniyot), such as a grandmother’s mother or a maternal uncle’s wife, were not invented by the rabbis but were authorized by God. Putting the pieces together, Saadia may have been arguing that if an obvious prohibition such as father-daughter incest requires tradition and cannot be created by a human jurist, then a more expansive restriction against the use of human reasoning to derive incest law certainly has merit.
Maimonides on the Source of the Prohibition
Moses Maimonides (1138–1204) offers a novel explanation for the “daughter gap” in his Book of the Commandments, where he found sources for each of the 613 biblical commandments. As the Talmud renders father-daughter incest a capital crime, it was clear to Maimonides that it must be a biblical law. As for its source, first, Maimonides notes that the prohibition is obvious, given the prohibition about granddaughters (akin to the a fortiori approach):
ספר המצוות, לא תעשה של"ו והמצוה השש ושלשים ושלש מאות, האזהרה שהוזהרנו מלבוא על הבת עצמה, וזה לא נתבאר בלשון התורה, לא נאמר לנו "ערות בתך לא תגלה." אלא שתק ממנה מחמת פשטות הענין והיותו ברור, לפי שאסר בת הבן ובת הבת הרחקה מן הבת.
Book of Commandments, Prohibitions #336 The three hundred and thirty sixth commandment is that we are prohibited against relations with one’s own daughter. This is not explicit in the Torah; there is no verse “do not uncover the nakedness of your daughter.” However, it was silent about this matter because it is clear and obvious, since it forbade the daughter of a son and the daughter of a daughter, who are even further from the daughter.
Maimonides continues by referencing the Talmudic passages (quoted above), saying that the real source of the prohibition is revelation:
ובגמר יבמות אמרו: בתו עיקר איסורא מדרשא אתיא... ולשון גמר כריתות : אל תהי גזרה שוה קלה בעיניך שהרי בתו אחד מגופי תורה ולא למדה הכתוב אלא בגזרה שוה...
In gemara Yevamot [3a] they said: “The basis of father-daughter incest is a midrash.” …  And the gemara of Keritot [5a] is: “Do not treat a gezeirah shavvah lightly, since father-daughter incest is one of the gufei torah and the Torah only taught it through a gezeirah shavvah.”…
But Maimonides is not satisfied that this passage will stand on its own. Instead, he offers his own spin on it:
והתבונן באמרם: "לא למדה הכתוב", ולא אמרו "לא למדנוה" לפי שכל הדברים האלה קבלה מפי השליח, והם פרוש מקובל, כמו שבארנו בהקדמת חבורנו בפרוש המשנה, ולא שתק הכתוב מלהזכירה אלא מפני שאפשר ללמדה בגזרה שוה. וזה הוא ענין אמרה לא למדה הכתוב אלא בגזרה שוה, ודי באמרם גוף תורה. 
Consider their statement “the Torah only taught it.” They did not say “they derived it” because all of these matters were transmitted from the emissary and it is a transmitted interpretation, as we explained in the Introduction to our work that comments on the Mishnah. However, scripture refrained from mentioning it because it may be known through a gezeirah shavvah. This is the meaning of their statement “the Torah only taught it through a gezeirah shavvah.” And their statement that it is guf torah is sufficient.
As Maimonides emphasizes, the passage in question has broad implications for his presentations of revelation. For him, this gezeirah shavvah is part of the corpus of knowledge that he called the “transmitted interpretation,” i.e., materials that God revealed to Moses in oral form. But there is even more going on here.
At the outset of his discussion, Maimonides announces that the Torah was silent about this matter because it is “obvious.” He thus mixes together the rabbinic a fortiori reasoning, the rabbinic gezeirah shavvah, and a new idea that certain laws can go unstated because any reader of the Torah could infer them from the existence of other laws.
Transport Impurity—More Obvious Law
Maimonides applies the idea of “obviousness” to other cases as well. According to Maimonides, the prohibition against father-daughter incest is so “clear and obvious” that the Torah need not even mention it. In his treatment of another nonscriptural law in his halakhic work, Mishneh Torah, he produces similar kinds of arguments, linking a fortiori reasoning with oral/aural tradition:
משנה תורה, ספר טהרה, הלכות טומאת מת א:ב טומאת משא מפי השמועה. וקל וחומר הדברים, אם נבילה שהיא טומאת ערב ואינה מטמאה באוהל, מטמאה במשא, שנאמר ״והנושא את נבלתם״, המת לא כל שכן....
Mishneh Torah, Book of Purity, Laws of the Corpse 1:2 Impurity transferred through transport is known by aural tradition. These matters are also a fortiori: if an animal carcass, which only transfers impurity until nightfall and does not transfer impurity within a structure nevertheless transfers impurity through transport—as it says, “one who carries their carcasses etc.” (Leviticus 11:28)—all the more so a human corpse!…
Maimonides continues by noting that this law should be obvious. For this reason, even though it lacks scriptural basis, it carries full biblical authority:
אין טומאת משא במת מדברי סופרים, אלא דין תורה.
Impurity for carrying a corpse is not rabbinic but biblical.
In order to substantiate this claim, Maimonides applies his novel claim about “obviousness”:
וייראה לי שזה ששתק ממנה הכתוב, כדרך ששתק מאיסור הבת לפי שאסר בפירוש אפילו בת הבת, ושתק מאיסור אכילת בשר בחלב לפי שאסר בפירוש אפילו בישולו, כך שתק מטומאת משא במת לפי שטימא בפירוש אפילו אהלו.
It seems to me that scripture was silent about this matter for the same reason that it was silent about father-daughter incest: because it explicitly forbade even a granddaughter. And it was silent about the prohibition against consumption of mixtures of milk and meat because it explicitly forbade even cooking it. So, too, it was silent about transportation of a corpse because it explicitly impurified even one who enters a structure with it—all the more so transporting.
What makes something biblical from the perspective of Maimonides is its inclusion in the Sinai revelation; whether it is explicit or implicit in the written Torah does not matter.
A Useful Gap
As was the case with Saadia and the Karaites, Maimonides used the “daughter gap” to illustrate his broader principles. For many medieval thinkers, the Torah’s silence about father-daughter incest prompted broad reflections about the formulation of scriptural rulings, scripture’s orally transmitted supplement, logical derivations from revelation, and hermeneutical reasoning more generally. The “daughter gap” thus provides a window into larger assumptions about the Hebrew Bible, among both medieval and among modern scholars.
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Dr. Marc Herman is an assistant professor in the Department of Humanities at York University and a core member of The Israel and Golda Koschitzky Centre for Jewish Studies. His Ph.D. is from the University of Pennsylvania’s Department of Religious Studies. He coedited Accounting for the Commandments in Medieval Judaism (2020) and has published in the Jewish Quarterly Review, Jewish History, and the Journal of the American Oriental Society, among other venues.
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