Gemara

5a - Question: What is Chachamim's source that an Aseh is Docheh a Lav?

Answer (Beraisa) Suggestion: Perhaps "A razor will not pass over Rosho" applies to a Nazir even if he is a Metzora!

Rejection: "Rosho" teaches that Tiglachas Metzora overrides the Lav for a Nazir to shave (his head).

Objection: The Isurim of a Nazir are weaker, since Yeshnam bi'Sh'eilah. We cannot learn to a regular Lav!

Support: There is also an Aseh for a Nazir to grow his hair ("Kadosh Yihyeh Gadel Pera". If we could learn from a Nazir, we would also learn that an Aseh overrides a Lav and an Aseh (and we know that it does not)!

7b (Beraisa): Normally, a Tevul Yom may not enter Har ha'Bayis. However, if Erev Pesach was the eighth day of Taharas Metzora, and he saw an emission that day and immersed, he may enter to complete his Taharah so he can offer Korban Pesach). It is better that an Aseh (Peach) punishable by Kares override an Aseh without Kares (Temei'im must leave the Mikdash).

(R. Yochanan): There is not even an Aseh of the Torah (for a Tevul Yom to leave. It is only mid'Rabanan.)

6a (Beraisa) Suggestion: If a father told his son (a Kohen) to become Tamei (Mes), or not to return a lost object, perhaps he should obey!

Rejection: "A man will fear his parents, and keep My Shabbosos" - you are all obliged to honor Hash-m.

Bava Metzi'a 30a (Beraisa): "V'His'alamta" - sometimes you ignore an Aveidah, e.g. if a Kohen sees an Aveidah in a cemetery, or a Chacham sees an Aveidah that it is undignified for him to carry it... he ignores it.

The verse is not needed for a Kohen who sees an Aveidah in a cemetery. Returning an Aveidah is an Aseh. It does not override the Aseh ("Kedoshim Yihyu") and Lav forbidding a Kohen to become Tamei.

Chulin 141a - Question (R. Aba brei d'Rav Yosef): If not for "Teshalach", we would permit taking a mother (resting on its chicks or eggs) for a Mitzvah. An Aseh and a Lav forbid this. An Aseh does not override them!

Answer: The case is, he already took the mother. He already transgressed the Lav. There is only an Aseh to send the mother. If not for "Teshalach," the other Aseh would override the Aseh to send.

Shabbos 132b - Rav Mesharshiya: "Besar" written regarding Milah on day eight permits a father who intends to cut his son's Tzara'as on the foreskin.

Question: If someone else (who is not concerned about the Tzara'as) can circumcise, the father should not be allowed!

(Reish Lakish): If one can fulfill an Aseh without transgressing a Lav, he must. If not, the Aseh overrides the Lav.

Answer: The case is, there is no one else to circumcise.

Rishonim

Question (Rif Bava Metzi'a 16b): The Gemara says that the Aseh of Hashavas Aveidah does not override the Aseh and Lav of Tum'as Kohanim. There is also a Lav not to ignore an Aveidah. Why didn't the Gemara mention this?

Answer (Rif): Only Mitzvos Aseh are Docheh, therefore the Lav of Hashavas Aveidah is irrelevant here. When the Gemara discusses whether or not Kibud Av va'Em is Docheh Hashavas Aveidah, it mentions the Lav and Aseh of Hashavas Aveidah, for both are relevant.

Hagahos me'Eretz Yisrael (2): The Lav is irrelevant because he does nothing. The act is only the Aseh, returning the Aveidah.

Question (Ha'Makneh on Tosfos Kidushin 34a DH Ma'akah, DH b'Emtza): It should be obvious that the Aseh and Lav of Hashavas Aveidah do not override the Aseh and Lav of Tum'as Kohanim!

Answer (Ha'Makneh, ibid.): The Ritva says that if the Aseh entails an action, the Lav is merely to strengthen the Aseh. If the Aseh is fulfilled through inaction, the Ikar is the Lav, and the Aseh is merely to strengthen the Lav. The Aseh and Lav of Tum'as Kohanim are both fulfilled passively, so one would have thought that they are like a Lav, which is Nidcheh for an Aseh.

Question: An Aseh does not override even an Aseh by itself. Why does it say 'an Aseh does not override a Lav and an Aseh'?

Answer #1 (based on Tosfos Chulin 141a DH Lo): An Aseh does not override a Lav and an Aseh, i.e. even the Lav is not overridden (totally. The Riva says that one is not lashed for it.)

Answers #2,3 (Ritva Bava Metzi'a 30a DH l'Mai): Indeed, this is not precise. Alternatively, since the Aseh of Hashavas Aveidah is Shavah b'Chol but the Aseh of Tum'ah is not, the former would override the latter if not for the Lav.

Ritva (Shabbos 132b DH Ika and Yevamos 6b DH Mai): When an Aseh is strong, it overrides an Aseh and a Lav. One opinion says that Taharas Metzora overrides Shilu'ach ha'Kan. Verses are needed to teach that Hashavas Aveidah does not override Tum'as Kohanim and Kibud Av v'Em does not override Shabbos.

Rambam (Hilchos Nezirus 7:15): If a Nazir became a Metzora and was cured of the Tzara'as, he shaves all his hair. Since it is impossible to fulfill the Aseh to shave and the Lav for a Nazir not to cut his hair, the Aseh is Docheh the Lav. A Nazir also has an Aseh to grow his hair, "Kadosh Yihyeh Gadel Pera". Everywhere, an Aseh is not Docheh a Lav and an Aseh. Here it is Docheh, for the days of Metzora Muchlat do not count towards Nezirus, so the Aseh "Kadosh Yihyeh" does not apply.

Rebuttal (Ra'avad): The Gemara in Yevamos (5a) says that Isurei Nazir are more lenient because Yeshnam bi'Sh'eilah.

Defense (Teshuvah Avnei Milu'im 22 DH v'Nir'eh): Mahari Basan says that Nezirus like a Neder; he forbids on his (appropriate) limbs wine, shaving and touching Tum'ah or entering Ohel ha'Mes. The Maharit holds that Kabalas Nezirus is Mekadesh the person, similar to a Kohen. Sugyos argue like these two opinions. The Rambam rules like the Maharit. The Torah put the Isurim on Nezirim, therefore they are Einam bi'Sh'eilah (one cannot annul the Isurim, only his Neder to be a Nazir).

Question: In any case, there is also an Aseh "K'Chol ha'Yotzei mi'Piv Ya'aseh"!

Answer #1 (Avnei Milu'im, ibid.): Since the Torah put the Isurim on Nezirim, "K'Chol ha'Yotzei mi'Piv Ya'aseh" does not apply.

Answer #2 (Minchas Chinuch 373 DH v'Ra'isi): If there is a Heter to shave, he does not transgress his vow.

Rambam (Hilchos Tzara'as 8:3): We shave around a Nesek and leave two hairs (all around) so it will be clear if it spread. This is even if the Metzora is a Nazir.

Question: In Hilchos Nezirus, the Rambam said that because the days of Metzora Muchlat do not count towards Nezirus, the Aseh "Kadosh Yihyeh" does not apply, therefore Tiglachas Metzora overrides the Lav. Here we discuss a Musgar, not a Muchlat, so this does not apply!

Answer #1 (Mishneh l'Melech): Toras Kohanim expounds "V'Hisgalach" (instead of v'Gilach) to permit even a Nazir.

Answer #2 (Aruch l'Ner Yevamos 5a DH v'Hinei): A tradition from Sinai teaches that shave around a Nesek is Docheh Nezirus. "V'Hisgalach" is not a real Drashah.

Tosfos (Chulin 141a DH Lo): The Gemara (Pesachim 47b) says that one who digs on Yom Tov is not lashed because the dirt could be used to cover the blood of a bird. The Riva asked, since Yom Tov is a Lav and an Aseh, an Aseh should not be Docheh them! He answered, in any case the Lav is Nidcheh. Regarding Shilu'ach ha'Kan (Chulin 141a), it says that he transgressed! Perhaps the Riva exempts from lashes, but agrees that he transgressed.

Sha'agas Aryeh (33): A Lav is more severe than an Aseh, nevertheless an Aseh is Docheh a Lav. An Aseh fulfilled passively is essentially a Lav. Why do we need a verse to forbid taking Em Al ha'Banim for Taharas Metzora? According to the Rambam, the Aseh of Shilu'ach is like a Lav, for he explains that the Aseh of Taharas Metzora does not override a Lav and an Aseh. If Shilu'ach was considered an Aseh of action, even when there is only the Aseh another Aseh would not override it! Rather, one would have thought that one may take the mother in order to fulfill both Mitzvos, i.e. it would be the bird sent away for Taharas Metzora.

Question: The opinion that Taharas Metzora permits him to his wife expounds "Teshalach" (to forbid even for this). According to the opinion that permits a Muhclat to his wife even before Taharah, what do we learn from Teshalach? The Gemara connotes that everyone expounds it!

Answer (Sha'agas Aryeh): The latter opinion must say that it teaches that here an Aseh of action overrides a passive Aseh, but everywhere else it does not. It cannot teach that everywhere an active Aseh is not Docheh a passive Aseh, for we would assume so without a verse. However, the former opinion holds that an active Aseh is never Docheh a passive Aseh; the verse teaches that Gadol ha'Shalom does not change this. The Halachah follows the latter opinion. This is why Yibum overrides Chayavei Aseh.

Note: If so, why was Rava refuted? He could say that the Beraisa which says that Yibum of a Petzu'a Daka acquires is according to the former opinion!

Teshuvas Noda bi'Yehudah (volume 1, Sof CM, DH Divrei): The Gemara never brings a source that an Aseh is not Docheh an Aseh. It is mere reasoning that one is not stronger than the other. It is better to transgress (one of them) passively than to transgress through an action. However, if he did an act to fulfill one of them, he was Yotzei. We do not say so regarding Yibum of Chayavei Lavin v'Aseh, since we expounded that there is a Yevamah who does Chalitzah but not Yibum.

Rebuttal (Teshuvas R. Akiva Eiger, Volume 3 (Chadashos) Siman 80): The Noda bi'Yehudah understands that mid'Oraisa, one may opt to fulfill either Aseh. The Gemara (Bava Metzi'a 30a and 32a) says that an Aseh is not Docheh an Aseh and a Lav, so we should not need verses to teach that one should not become Tamei to return an Aveidah, nor listen to one's father to transgress! According to the Noda bi'Yehudah, we need a verse to teach that one may not choose!