Commentaire sur Le Lévitique 18:10

ערות בת בנך THE NAKEDNESS OF THE DAUGHTER OF THY SON [OR THE DAUGHTER OF THY DAUGHTER THOU SHALT NOT UNCOVER] — When it states "thy daughter״ Scripture is speaking of his daughter born of a woman whom he had outraged (similarly "the daughter of thy son" means, the son born of such a woman). This must be so, for that one is forbidden to marry one's daughter or the daughter of one's daughter who was born of one's wife we may learn from the prohibition (v. 17) in the verse beginning אשה ובתה ערות "the nakedness of a woman and her daughter" of whom it is stated "thou shalt not uncover" and which thus implies the prohibition to marry one's daughter (and forbids one's marriage with one's daughter's daughter; for Scripture continues there: “neither shalt thou take her son's daughter or her daughter's daughter) whether she be begotten by him or by another man (cf. Yevamot 22b and Rashi von v. 17).

ערות בת בנך או בת בתך לא תגלה, “You must not expose the nakedness of your son’s daughter or you daughter’s daughter, etc.” Interestingly, the Torah failed to mention the prohibition of sexual relations with one’s daughter; the reason is that this may be arrived at by simple logic. If the Torah provides a harsh penalty for cohabiting with one’s granddaughter, the same is most certainly appropriate for a father who cohabits with his daughter. The absence of the direct prohibition of sleeping with one’s daughter is comparable to the direct prohibition of eating meat and milk. Seeing that the Torah forbade the boiling of meat in milk, (Exodus 23,19) it is clear that eating such a mixture is most certainly forbidden also.
In connection with Exodus 23,19 I explained the reason why the Torah phrased that prohibition as applying to the boiling of the two items of food instead of simply forbidding the consumption of the two at the same time. Seeing that the Torah failed to mention “do not uncover the nakedness of your daughter,” our sages (Yevamot 3) concentrated on a different method of deriving this prohibition by insisting that there is a written link to this legislation in the Torah called גזרה שוה, the use of otherwise redundant words which contribute little to our understanding where they appear. The words in question are זמה,זמה and הנה,הנה respectively (verse 17 and chapter 20,14 ). [The sages were not happy with relying on logic, i.e. a קל וחומר, seeing that such a source for a prohibition, while adequate, cannot be used to determine that the prohibition carries such a harsh penalty as karet. Ed.]

His daughter [from a woman he] raped. Rashi is answering the question: Since it is written “your son’s daughter or your daughter’s daughter,” this implies that the daughter of a son or daughter she had from another man is permitted. But it is written (v. 17), “The nakedness of a woman and her daughter you shall not uncover. The daughter of her son and the daughter of her daughter”! Therefore Rashi explains: [Over here] we are dealing with a raped woman who is not his wife. Therefore [regarding her children] there is only a prohibition against the daughter of his daughter who came from his seed. But the daughter of the woman he raped came from the seed of other men, she is not included in the prohibition of “you shall not uncover” [in our verse]. And the same law applies to the daughter of his wife who is not his daughter.