הלכה על יבמות 156:6

The laws of this commandment - meaning to say how did they, may their memory be blessed, learn it and from which verse - are in the Gemara Yevamot. As there (Yevamot 3a) they said, "The main prohibition of his daughter from a woman he raped, comes by interpretation (drasha), as Rava said [that] Rav Yitzḥak bar Avdimi said to [me], 'This prohibition comes [by means of an inferential comparison between] "their" (hena) and "their"; it comes [by an inferential comparison of] "licentiousness" and "licentiousness"'" - meaning to say, that the verse states with the son’s daughter and the daughter’s daughter, "it is their nakedness"; like it is written at the end of the section (Leviticus 18:17), "their flesh." Just like over there, it is explicit that her daughter is forbidden; so too here, wherein the verse prohibits the daughter of his son, the law is the same for his daughter even though the verse did not explain it - as we learn it from this inferential comparison. And they, may their memory be blessed, also learned with which death one who has intercourse with his daughter or with the daughter of his son or the daughter of his daughter is judged, from the strength of this inferential comparison. They, may their memory be blessed, learned it from it, after it was learned about it from another place. And from what other place was it learned about it? From that which is written (Leviticus 20:14), "And a man that takes a woman and her mother, it is licentiousness; with fire shall they be burnt, him and them." And they, may their memory be blessed, said (Sanhedrin 75b) [that] just like there, with a woman and her mother, about them which is written, "licentiousness," it is with burning; so too here, with a woman and her daughter or the daughter of her son or the daughter of her daughter, about them which is written, "their flesh, it is licentiousness," he is judged with burning. And from now, since we found that the judgement of one who has intercourse with a woman and her daughter or the daughter of her son or the daughter of her daughter is with burning - since we have already learned the prohibition of his daughter from it by the inferential comparison of "their, their"; we can further learn from all of the other laws in it, and say that the judgement of one who has intercourse with his daughter is also with burning. And about that which is similar to this, they say in the Gemara (Yevamot 78b), "Infer from it and from it" - meaning to say, when we learn one verse from its fellow, we do not learn it for only one thing about it, but rather we learn it for all its laws from it. And [this is the case] even when those laws that are with it are not like the understanding of that verse itself, but rather learned from other verses. Nonetheless, we learn from everything that is in it - whether from the verse itself or whether it is learned from another place. And they, may their memory be blessed, said in the Gemara Keritot 5a, "Let not an inferential comparison (gezara shava) be light in your eyes; as behold, his daughter from a woman he raped is one of the [important] bodies of Torah, and Scripture only taught it through a gezara shava - it comes by 'their, their'; 'licentiousness, licentiousness.'" [This is] meaning to say that we learned its prohibition and its judgement from these two inferential comparisons, as we explained. But his daughter from a woman he married is explicit in Scripture, as it is stated (Leviticus 18:17), "The nakedness of a woman and her daughter" - and there is no distinction whether she is his daughter and her daughter, or from another man. And this is speaking about a married woman, since it is written, "woman" (eeshah, which is also the word for wife), which implies the language of marriage (eeshut). And it is also written "you shall not take" - and taking also implies through marriage. But regarding his daughter from a woman he raped or the daughter of her son or daughter of her daughter, only, "you shall not reveal," is written. And Rambam, may his memory be blessed, wrote (in Sefer HaMitzvot LaRambam, Mitzvot Lo Taase 336), "Observe their, may their memory be blessed, saying 'The verse did not teach it,' and they did not say, 'We did not learn it' - since all of these matters are a transmission from the messenger (Moshe), peace be upon him, who transmitted the understanding [of the Torah] to the elders. And that is [the meaning] of their saying, 'body of Torah,' about this." And [this] brought the rabbi, may his memory be blessed, to write this as a fixed major principle for himself - that only what is explicit in the verse or that which they, may their memory be blessed, said explicitly that it is from the Torah, is in the tally of the six hundred and thirteen commandments, but not that which we learn from the thirteen hermeneutic principles. And Ramban, may his memory be blessed, already contradicted this with clear proofs (in Sefer HaMitzvot LaRambam, Root 2).