1) DERIVING A PUNISHMENT THROUGH A "KAL VA'CHOMER"
QUESTION: The Gemara suggests that when another person causes a Nazir to become Tamei, that person transgresses the Isur of Tum'ah of a Nazir, like the Nazir himself who becomes Tamei. The Gemara suggests that the source for this is a Kal va'Chomer from the Isur of shaving. When another person shaves the head of a Nazir, that person transgresses the Isur even though the shaving of a Nazir is not "Soser" the entire Nezirus. Certainly, then, the person who is Metamei a Nazir should be liable for transgressing the Isur of Tum'ah, since Tum'ah is "Soser" the entire Nezirus.
Why does the Gemara say that a Kal va'Chomer is needed to teach this? If a person is Metamei a Nazir, he should be liable for transgressing a Lo Ta'aseh even without the Kal va'Chomer. The Torah teaches (Vayikra 21:1) that one may not actively be Metamei a minor Kohen, under the age of thirteen (see Yevamos 114a). This prohibition certainly should apply to an adult Kohen -- and to a Nazir -- as well, and thus one should be prohibited from actively causing him to become Tamei. Why is it necessary for the Gemara to derive through a Kal va'Chomer that one is prohibited to be Metamei a Nazir? He is prohibited to be Metamei a Nazir because of the verse the Gemara cites in Yevamos!
Perhaps the verse cited in Yevamos teaches only a Mitzvas Aseh. The Gemara here derives through the Kal va'Chomer that the Lo Ta'aseh of "Lo Yitama" also applies, and that one who is Metamei a Nazir is punishable with Malkus.
However, this answer is not viable. The principle of "Ein Onshin Min ha'Din" teaches that a punishment (like Malkus) may not be administered based on a Limud from a Kal va'Chomer.
What is the purpose of the Kal va'Chomer? (KEREN ORAH)
(a) The KEREN ORAH answers that the Gemara simply intends to derive from a Kal va'Chomer that one who is Metamei a Nazir transgresses two Isurim. (The Gemara in Yevamos (33b) explains that the practical consequence of transgressing a "double" Isur is that the person "will be buried among Resha'im Gemurim," the utterly evil.)
(b) Another answer may be suggested based on the words of the MAGID MISHNEH (Hilchos Ma'achalos Asuros 2:1). The RAMBAM there writes that the Torah prohibits (with an Isur Lav) the consumption of four specific types of animal which lack one sign of Taharah, and "all the more so" a person who eats an animal which lacks both signs of Taharah receives Malkus for transgressing the Isur. Why does the Rambam write that he is punished with Malkus because of a Kal va'Chomer, if "Ein Onshin Min ha'Din"? The Magid Mishneh answers that the Rambam there also writes that one who eats any animal which lacks one of the two signs of Taharah transgresses an Isur Aseh as well. Since the consumption of such an animal is already prohibited by an Isur Aseh, the Kal va'Chomer is able to teach that one receives Malkus for it. (That is, when the act is already prohibited by an Isur, a Kal va'Chomer can be used to administer a punishment. See MISHNEH L'MELECH and SHA'AR HA'MELECH.)
The source for the Magid Mishneh's answer seems to be the Sifri cited by the Rambam in Sefer ha'Mitzvos (Lo Ta'aseh 172). The Sifri mentions the Kal va'Chomer of the Rambam and concludes that the Isur Aseh against eating such animals is explicitly mentioned in the verse, and the punishment of Malkus is derived through the Kal va'Chomer.
Similarly, the Gemara here perhaps seeks to derive through a Kal va'Chomer that a person who is Metamei a Nazir receives Malkus, since the verse itself teaches an Isur Aseh.
What is the logic behind the Magid Mishneh's assertion that "Ein Onshin Min ha'Din" does not apply in such a case? Why should a Kal va'Chomer be able to teach that a punishment is administered when the Isur is already known?
The Rambam himself proposes a logic basis for this in Sefer ha'Mitzvos (Lo Ta'aseh 172). The Rambam writes that administering Malkus based on a Kal va'Chomer in such a case is not included in the principle of "Ein Onshin Min ha'Din" because this Kal va'Chomer is only a "Giluy Milsa b'Alma" (see Sanhedrin 76a) and not a full-fledged Kal va'Chomer. It is similar to the Kal va'Chomer which teaches the Isur against living with one's daughter, which is derived from the verse which prohibits living with one's granddaughter.
This may be understood as follows. According to some Mefarshim, the reasoning behind the principle of "Ein Onshin Min ha'Din" is that a Kal va'Chomer is derived based on logic (and not based on tradition). Since the logic upon which the Kal va'Chomer is based might be flawed, one cannot be fully confident that his Kal va'Chomer is correct such that corporal punishment may be inflicted on a transgressor because of it. However, once there is strong reason to assume that the Kal va'Chomer is valid, a person may be punished based on it. This is the role of a Kal va'Chomer which acts as a "Giluy Milsa." The Kal va'Chomer which teaches the prohibition against living with one's daughter from the prohibition against living with one's granddaughter is so obvious that a person may be punished based on it. Similarly, if it is clear that the Torah intends to prohibit a certain act because it gives an Isur Aseh, and a similar act is prohibited with a Lo Ta'aseh as well, a Kal va'Chomer is able to serve as a "Giluy Milsa" to teach that the first act is also prohibited with a Lo Ta'aseh (and hence is punishable with Malkus).