Okay, this next piece is going to be pretty interesting. Get ready because it’s really different.
Feel free to read through Leviticus chapter 18, which lists the forbidden sexual relationships called the “arayos.” Find the verse that prohibits one to have relations with his daughter. I’ll wait.
If you couldn’t find it – and I’ll assume that you couldn’t – it’s because there isn’t one. (Well, there is one, it’s just not stated in the text.)
You see, verse 18:10 prohibits sexual relations with a granddaughter, regardless of whether she’s the daughter of one’s son or one’s daughter. Having said that, the prohibition against having relations with one’s daughter is pretty obvious. It’s so obvious that the Torah doesn’t even need to state it outright, yet it’s still one of the 613 mitzvos!
There are 13 methodologies of Torah exegesis, outlined for us by Rabbi Yishmael. (We recite them every day in the preliminary morning prayers.) The first and most basic is called a kal v’chomer, usually translated by the Latin phrase “a fortiori.” An argument “a fortiori” is one that proves a case based on the fact that a more difficult case has already been proven. If I’ve demonstrated that I can lift 100 pounds, you know a fortiori that I can lift 50 pounds.
So, having prohibited both a son’s daughter and a daughter’s daughter on the basis that such relationships are incestuous, kal v’chomer (a fortiori), sexual relations must be prohibited with one’s own daughter! QED.
In addition to the kal v’chomer, there is also a gezeira shavah, that is, a tradition of similar laws applying to cases with similar terminology. The practical difference is that something might be prohibited because of a kal v’chomer but a person would not be liable to punishment for its transgression. Without the gezeira shavah, incest with a daughter would be prohibited but not actionable. (All this is from Talmud Sanhedrin 76a; see there for full details.)
If a person had any doubts as to the 13 Torah-learning methodologies – called the hermeneutical rules – preferring to take the text strictly at face value, this mitzvah should clear things up. The alternative is that the Torah prohibits a stepmother or an aunt as incestuous relationships but not one own’s daughter. Such a conclusion is inconceivable!
(Incidental to all this, there is a prohibition on relations with both a mother and her daughter. If the man is her father, he obviously had relations with her mother. We’ll see more about that in Mitzvah #203.)
The reason for this mitzvah is what we explained in the previous arayos, about the preventive measures taken among family members, with whom we interact constantly and intimately.
This mitzvah applies in all times and places. It is discussed in the Talmud in the tractates of Yevamos (3a) and Sanhedrin (76a) and is codified in the Shulchan Aruch in Even Ha’Ezer 15. This prohibition is #336 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #120 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.