1) EXPOUNDING "SEMUCHIN" TO TEACH "ASEH DOCHEH LO TA'ASEH"
QUESTION: The Gemara discusses at length the source for the principle of "Aseh Docheh Lo Ta'aseh." It proposes that this principle is derived from the proximity of the prohibition against wearing Kil'ayim with the command to make Tzitzis for one's garment. The proximity (Semuchin) of these two verses teaches that one may place threads of wool on a garment of linen in order to make Tzitzis, and that the Mitzvas Aseh of Tzitzis overrides the Lo Ta'aseh of Kil'ayim.
The Gemara concludes that this is a valid source for "Aseh Docheh Lo Ta'aseh" according to d'Vei Rebbi Yishmael who maintains that the verse of Kil'ayim contains an extra phrase which gives us license to apply Semuchin and to derive that the Aseh of Tzitzis overrides the Lo Ta'aseh of Kil'ayim. According to the Rabanan, however, there is no extra phrase in the verse (because they maintain that the phrase "Tzemer u'Pishtim" is necessary to teach that the prohibition of Kil'ayim applies only to a mixture of wool and linen and not to any other mixture). The Gemara asks what is the source, according to the Rabanan, for the principle of "Aseh Docheh Lo Ta'aseh," since Semuchin can be applied only when there is an extra phrase in the verse.
What is the Gemara's question? It is only Rebbi Yehudah who requires an extra phrase ("Mufneh") in the verse in order to expound Semuchin. The Rabanan who argue with d'Vei Rebbi Yishmael may follow the view of the Rabanan who argue with Rebbi Yehudah and do not require an extra phrase in order to expound Semuchin.
ANSWERS:
(a) TOSFOS (DH l'Rabanan) says that the Gemara asks this question merely to cover all possibilities, including the possibility that the Rabanan agree with Rebbi Yehudah.
(b) The RITVA answers that the Rabanan -- who disagree with Rebbi Yehudah and apply Semuchin even when there is no extra phrase in the verse -- nevertheless require an extra phrase to extend the law derived from Semuchin to other cases through a Binyan Av. When there is no extra phrase, the law derived through Semuchin applies only to the specific case mentioned in the verse and cannot be applied to other cases. (See also ME'IRI)
(c) The TOSFOS HA'ROSH in the name of RABEINU MEIR explains that even if the Rabanan maintain that we expound Semuchin, perhaps in this case the Semuchin does not teach that one may place Tzitzis of wool on a garment of linen. Perhaps according to the Rabanan (who do not maintain that whenever the verse says "Beged" it refers exclusively to wool and linen) the Semuchin teaches a different Halachah altogether -- the Halachah of Rava, that Tzitzis of wool be may placed on a garment of any material (including linen), but Tzitzis of linen may be placed only on a garment of linen or other non-wool material (and Tzitzis made of any other material may be placed only on a garment made of the same material).
2) THE TEACHING OF THE VERSE OF "ROSHO"
QUESTION: The Gemara explains that the verse, "Rosho" -- "On the seventh day he shall shave off all of his hair, [and the hair of] his head" (Vayikra 14:9), teaches that the Mitzvas Aseh for a Metzora to shave his hair overrides the Lo Ta'aseh against shaving the hair of the corners of one's head ("Hakafas ha'Rosh"). The Gemara suggests that this is the source for the principle of "Aseh Docheh Lo Ta'aseh." The Gemara (5b) concludes that there is another source for "Aseh Docheh Lo Ta'aseh" -- the verses which teach that the Mitzvas Aseh of Tzitzis overrides the Lo Ta'aseh of Kil'ayim.
If another source teaches that an Aseh overrides a Lo Ta'aseh, why is the verse of "Rosho" necessary? Once the Torah teaches that an Aseh overrides a Lo Ta'aseh, that principle may be applied to a Metzora who is obligated by a Mitzvas Aseh to shave his entire head, even though a Lo Ta'aseh prohibits one from shaving his entire head. If an Aseh overrides a Lo Ta'aseh in the case of Tzitzis and Kil'ayim (where the Lo Ta'aseh applies to everyone), then in the case of a Metzora -- where the Lo Ta'aseh against shaving the entire head applies only to men and not to women -- the Aseh certainly should override the Lo Ta'aseh. (The Gemara in Nazir 58a implies that according to the Tana who expounds the verse of "Rosho," the source for "Aseh Docheh Lo Ta'aseh" is not the verses regarding Tzitzis, but the word "Rosho" in this verse. The Sugya here, however, does not agree with that Tana.)
ANSWERS:
(a) TOSFOS (DH Mah l'Lav) answers that the verse of "Rosho" is needed to teach a different Halachah and not the principle of "Aseh Docheh Lo Ta'aseh." It teaches that shaving the entire head is considered "Hakafah" ("Hakafas ha'Rosh Shmei Hakafah") and it would have been prohibited had it not been for the Mitzvas Aseh.
(b) Alternatively, without the verse of "Rosho" one might have thought that the second verse quoted by the Gemara -- the verse of "Zekano" -- does not refer to the Mitzvas Aseh of a Kohen Metzora to shave his hair and does not teach a new principle of "Aseh Docheh Lo Ta'aseh v'Aseh" in the case of a Kohen Metzora. Rather, one might have thought that the verse of "Zekano" refers to the Mitzvas Aseh of an ordinary Metzora to shave his hair, a case in which there is only a Lo Ta'aseh not to shave, and thus the Aseh certainly overrides the Lo Ta'aseh. The verse of "Rosho" teaches that "Zekano" indeed refers to a Kohen Metzora and teaches that an Aseh overrides both an Aseh and a Lo Ta'aseh.

5b----------------------------------------5b

3) A POTENTIAL SOURCE FOR "ASEH DOCHEH LO TA'ASEH SHE'YESH BO KARES"
QUESTION: 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.
How does the Gemara know that an Aseh overrides a Lo Ta'aseh which is punishable with Kares? There seems to be evidence to the contrary. The fact that the Torah specifically teaches, in the case of Tzitzis made from Kil'ayim, that an Aseh overrides an ordinary Lo Ta'aseh (one which is not punishable with Kares) implies that an Aseh does not override a Lo Ta'aseh which is punishable with Kares. Why, then, does the Gemara seek a source to prove that it does?
ANSWERS:
(a) TOSFOS (6a, DH Ta'ama) explains that one might have thought that the verse of Tzitzis does not teach that an Aseh overrides a Lo Ta'aseh, but rather it teaches the Halachah of Rava, that Tzitzis of wool or linen may be placed on a garment of any material. Without the verse, one would have thought that the Tzitzis must be made from the same material from which the garment is made. Since the verse is needed to teach that the garment may be made from a different material than the Tzitzis, one might have assumed that the verse cannot be used to teach "Aseh Docheh Lo Ta'aseh." Consequently, the Gemara is justified in seeking another source that teaches that an Aseh overrides a Lo Ta'aseh that has Kares.
(b) TOSFOS further suggests that perhaps the Mitzvas Aseh of Tzitzis is considered a weaker Aseh because it does not apply to everyone (women are exempt from the Mitzvah of Tzitzis). For this reason, a verse is needed to teach that the Mitzvah of Tzitzis nevertheless overrides the Lo Ta'aseh of Kil'ayim.
(This answer does not explain why the Gemara attempts to derive from the Mitzvah of Milah that an Aseh overrides a Lo Ta'aseh that has Kares. Milah also is a Mitzvah which does not apply to everyone, and nevertheless it overrides a Lo Ta'aseh that has Kares. Accordingly, the Mitzvah of Tzitzis certainly should override an ordinary Lo Ta'aseh.) (See MELO HA'RO'IM to 6a for another possible answer.)

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