1) DOES A "HECHSHER MITZVAH" OVERRIDE A "LO TA'ASEH"
OPINIONS: The Gemara seeks a source for the principle that "Aseh Docheh Lo Ta'aseh she'Yesh Bo Kares," a Mitzvas Aseh overrides even a Lo Ta'aseh which is punishable with Kares. The Gemara suggests that this principle is derived from the Mitzvah of Kibud Av va'Em. The Torah teaches that the Mitzvah of Kibud Av va'Em does not override the Lo Ta'aseh not to perform Melachah on Shabbos (for example, in a case in which one's parent tells him to desecrate Shabbos), which is a Lo Ta'aseh which is punishable with Kares. The fact that the Torah specifically states that Kibud Av va'Em does not override the Lo Ta'aseh of Shabbos implies that in all other cases an Aseh does override a Lo Ta'aseh that has Kares.
The Gemara rejoins that no proof may be deduced from Kibud Av va'Em, because perhaps when the Torah says that Kibud Av va'Em does not override the Lo Ta'aseh of Shabbos, it refers only to a Lo Ta'aseh of Shabbos which is not punishable with Kares, such as the prohibition of Mechamer (leading an animal). Accordingly, no inference can be made from that case to the case of a Lo Ta'aseh that has Kares.
The Gemara asks that if the verse refers only to a Lo Ta'aseh of Shabbos which is not punishable with Kares, then the opposite law should be derived from there -- that an Aseh does not override even an ordinary Lo Ta'aseh. The Gemara proposes that the Lo Ta'aseh of Shabbos differs from other prohibitions and is more stringent, but the Gemara refutes that answer.
The Gemara concludes that since Kibud Av va'Em is a "Hechsher Mitzvah," it differs from other Mitzvos Aseh and does not override a Lo Ta'aseh.
What does the Gemara's answer mean? What is a "Hechsher Mitzvah," and why is the Mitzvah of Kibud Av va'Em considered one? Why does such a Mitzvah not override a Lo Ta'aseh?
(a) RASHI understands that "Hechsher Mitzvah" in this context refers to a Mitzvah which is fulfilled only by transgressing a Lo Ta'aseh. There is no way to fulfill the Aseh without transgressing the Lo Ta'aseh.
Rashi understands that the Gemara returns to its original assumption that the verse refers to a Lo Ta'aseh of Shabbos which is punishable with Kares. The verse teaches that the Mitzvah of Kibud Av va'Em does not override a Lo Ta'aseh of Shabbos that has Kares even though it is a "Hechsher Mitzvah" (and in all other cases of a "Hechsher Mitzvah" the Mitzvah does override a Lo Ta'aseh that has Kares). Without the verse, one would have thought that Kibud Av va'Em also overrides Shabbos since the fulfillment of the parent's request (to cook for him on Shabbos, for example) is not possible without also transgressing the Lo Ta'aseh. In contrast, in the case of Yibum with a woman who is an Ervah to her brother-in-law, it is possible to avoid the Lo Ta'aseh that has Kares and fulfill the Mitzvah by doing Chalitzah instead.
In short, according to Rashi, the Gemara means that an Aseh can override a Lo Ta'aseh that has Kares when there is no other way to fulfill the Aseh (in this particular situation) other than by transgressing the Lo Ta'aseh.
The Gemara's answer, however, is still difficult to understand. In practice, there is never a case in which an Aseh overrides a Lo Ta'aseh which is punishable with Kares.
Moreover, the Gemara in Bava Metzia (32a) points out that aside from the fact that an Aseh does not override a Lo Ta'aseh that has Kares, there is another reason for why the Mitzvah of Kibud Av v'Em (or Binyan Beis ha'Mikdash) obviously does not override Shabbos. The observance of Shabbos involves both an Aseh and a Lo Ta'aseh, and a Mitzvas Aseh cannot override an Aseh and Lo Ta'aseh together (see TOSFOS DH Nigmar). The Gemara in Bava Metzia answers that the Aseh of Kibud Av va'Em is stronger than any other Aseh, since the honor of one's parents is compared to the honor of Hash-m, and thus one might have thought that it overrides even an Aseh and a Lo Ta'aseh together. (The same can be said of the Mitzvah of Binyan Beis ha'Mikdash, as the awe for the Beis ha'Mikdash is compared to the awe of the Almighty.)
Why, though, does the Gemara assert that we cannot learn from Kibud Av va'Em that every Mitzvah overrides a Lo Ta'aseh that has Kares because it is a "Hechsher Mitzvah"? The Gemara should say that we cannot compare any other Mitzvas Aseh to Kibud Av va'Em because Kibud Av va'Em is stronger than any other Mitzvah! The MAHARSHA suggests that the Gemara indeed could have given that answer. Kibud Av va'Em is not only a "Hechsher Mitzvah" but an especially strong Mitzvah in its own right. Hence, even according to the Gemara's conclusion we cannot learn from Kibud Av va'Em that an ordinary Aseh overrides a Lo Ta'aseh that has Kares when there is no way to avoid the Lo Ta'aseh.
(b) TOSFOS explains that "Hechsher Mitzvah" here means the opposite of what Rashi says: an Aseh which is only a "Hechsher Mitzvah" does not override a Lo Ta'aseh even if the Lo Ta'aseh is not punishable with Kares (such as Mechamer on Shabbos). A "Hechsher Mitzvah" refers to an act which is merely preparatory to the Mitzvah and not the actual fulfillment of the Mitzvah itself. When a person leads an animal on Shabbos in order to bring food to his father, the act of Mechamer is only a preparatory act for the fulfillment of the Mitzvah. The actual Mitzvah is fulfilled when one gives the food to his father.
According to Tosfos, the Gemara does not return to its original assumption. Rather, the Gemara upholds its explanation that the verse refers to a Lo Ta'aseh of Shabbos that does not have Kares (such as Mechamer), and it teaches that since the act of transgressing Mechamer is only preparatory to the main Mitzvah of Kibud Av va'Em, one may not transgress a Lo Ta'aseh (even one that does not have Kares) in order to fulfill the Mitzvah of Kivud Av va'Em.
Rashi apparently rejects this explanation because Mechamer is not always a mere act of preparation for the Mitzvah of Kibud Av. When one's father specifically instructs his son to guide the animal for him on Shabbos, the act of Mechamer is the actual fulfillment of the Mitzvah and is not just a preparatory act.
Tosfos addresses this question. He explains that although in some situations the act of Mechamer may be the actual fulfillment of the Mitzvah of Kibud Av va'Em, nevertheless in most situations Mechamer is merely a preparatory act. For this reason, even when the father instructs his son to guide the animal on Shabbos, the Mitzvah of Kibud Av does not override the Lo Ta'aseh of Mechamer. According to Tosfos, the Gemara means that an act which is usually an act of preparation for a Mitzvah, "Hechsher Mitzvah," does not override a Lo Ta'aseh.

6b----------------------------------------6b

2) A CASE OF AN "ASEH" WHICH DOES NOT OVERRIDE A "LO TA'ASEH"
QUESTIONS: The Gemara derives from a verse that Beis Din is not allowed to kindle a fire on Shabbos (Hav'arah) in order to kill a person who is liable for Sereifah. One might have thought that Hav'arah in such a situation is permitted because of the principle of "Aseh Docheh Lo Ta'aseh she'Yesh Bo Kares." The Gemara concludes that Hav'arah on Shabbos is a Lo Ta'aseh that does not have Kares. Since the Torah already teaches that an Aseh overrides an ordinary Lo Ta'aseh (one that does not have Kares), a verse is needed to teach that in the case of Hav'arah on Shabbos, the Mitzvas Aseh of Misas Beis Din does not override the Lo Ta'aseh of Hav'arah.
(a) If Hav'arah is only a Lo Ta'aseh, then the Gemara should learn from this case that an Aseh does not override an ordinary Lo Ta'aseh (as the Gemara asks earlier (6a) with regard to Mechamer). Why does the Gemara here not make such an inference?
(b) Why is a verse needed to teach that the Aseh does not override the Lo Ta'aseh in this case? The principle of "Aseh Docheh Lo Ta'aseh" obviously does not apply in this case because the Aseh is not performed at the same time as the Lo Ta'aseh. The Lo Ta'aseh occurs when the fire is kindled and the lead is melted, while the Aseh is fulfilled only when the person is killed.
(c) Why does the Gemara say that kindling a flame on Shabbos in order to fulfill the Mitzvah of Sereifah constitutes only a Lo Ta'aseh that does not have Kares? The act also involves the Melachah of "Netilas Neshamah" -- killing a living creature on Shabbos, which is a Lo Ta'aseh that has Kares.
ANSWERS:
(a) The Rishonim suggest two approaches to the first question.
1. The RASHBA explains that kindling the fire in this case is only a "Hechsher Mitzvah" for the actual Aseh of killing the person, and that is why it does not override a Lo Ta'aseh (as the Gemara teaches on 6a). (The Rashba understands "Hechsher Mitzvah" as Tosfos explains it; see previous insight.) The Gemara could have asked that no verse is needed to teach this since it is derived from another verse (6a), but the Gemara had a better question to ask.
2. TOSFOS and other Rishonim explain that on Shabbos, in addition to the Lo Ta'aseh against desecrating Shabbos there is also an Aseh to guard and observe the laws of Shabbos. That Aseh applies to all of the prohibitions of Shabbos, and thus any act of Shabbos desecration involves the violation of both an Aseh and a Lo Ta'aseh. Accordingly, one who performs the act of Hav'arah transgresses both an Aseh and a Lo Ta'aseh (according to Rebbi Yosi). This explains why the Aseh does not override the Lo Ta'aseh in this case; an Aseh cannot override both another Aseh and a Lo Ta'aseh. Hence, this case does not prove that an Aseh does not override a lone Lo Ta'aseh. (Why, though, does the law in this case not imply that in all other cases of an Aseh together with a Lo Ta'aseh, an Aseh does override both? Tosfos writes that the Gemara could have asked this question, but it had a better question to ask.)
(b) Three approaches are suggested to answer the second question.
1. The RASHBA writes that the Gemara indeed could have asked why a verse is needed to teach that the Aseh does not override the Lo Ta'aseh in this case, when, in this case, the Aseh is not performed at the same time as the Lo Ta'aseh.
2. The ARUCH LA'NER explains that perhaps since the Aseh of killing the perpetrator is a "Mitzvah d'Rabim," a public Mitzvah incumbent upon all of the Jewish people, it is a "stronger" Mitzvah than others (see TOSFOS 6a, DH Nigmar) and thus it can override a Lo Ta'aseh even when they are not done at the exact same time.
3. The RAMBAN writes that it is possible to perform the Aseh and the Lo Ta'aseh at the same time by holding the lead above the perpetrator's mouth and burning it there so that it falls directly into her mouth and kills her. This is considered "at the same time."
The RASHBA rejects this answer because the two events still occur at two separate times. The lead does not go into the mouth at the same time it is kindled; one event follows the other sequentially.
Perhaps the Ramban follows the view expressed by the NIMUKEI YOSEF (Bava Metzia 32a) who explains that when the beginning of an act which fulfills a Mitzvas Aseh is done at the same time as the Lo Ta'aseh, the two are considered to occur "at the same time." If the lead is held above the open mouth of the Bas Kohen, one begins the act which kills her when he lights the fire which melts the lead which drips into her mouth. The Rashba, on the other hand, maintains that the Aseh must be completely fulfilled at the time the Lo Ta'aseh is transgressed in order to override it, but in this case the Aseh is fulfilled only at the moment the Bas Kohen dies (and not when the lead is poured into her mouth).
(c) Why does the Gemara not ask that the kindling of a flame on Shabbos in order to fulfill the Mitzvah of Sereifah constitutes the Lo Ta'aseh of "Netilas Neshamah," which is a Lo Ta'aseh that has Kares? The RASHBA (TESHUVOS 1:357) writes that this essentially is what the Gemara asks when it says that there still is a Meleches Shabbos involved with the act. The Gemara mentions the Melachah of Bishul, but it could have mentioned Netilas Neshamah as well.
The Rashba adds, however, that the wording of the Beraisa (and the verse) implies that the prohibition being discussed is that of making a fire and burning the lead, not the prohibition against killing on Shabbos.
Perhaps the intention of the verse is different, and it does not intend to teach that the Aseh of Sereifah does not override the Lo Ta'aseh of Hav'arah. One would not have thought that a Jew may execute a guilty Bas Kohen on Shabbos, such that a verse is needed to teach that he may not perform the execution. Rather, one would have thought that a Jew is permitted to heat the lead with which a Nochri will execute the Bas Kohen on Shabbos. The verse teaches that even the burning of the lead is prohibited and may not be done by a Jew on Shabbos. Accordingly, the Gemara's question is logical. The Gemara asks why one would have thought that burning the lead is permitted on Shabbos, when that act constitutes not only a transgression of Hav'arah but also a transgression of Bishul, which is punishable with Kares.

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