1) WOMEN READING THE MEGILAH FOR MEN
QUESTION: Rebbi Yehoshua ben Levi rules that women are obligated in the Mitzvah of Mikra Megilah. Tosfos points out that even though the Gemara says that women are obligated in the Mitzvah, the Tosefta says that they may not read the Megilah for men.
If women are obligated in the Mitzvah, why may they not read the Megilah for men?
ANSWERS:
(a) The RITVA says that the words of the Tosefta are in error. The Gemara says clearly that women are obligated in the Mitzvah, and thus they may read the Megilah for men. This is also the view of RASHI in Erchin (3a, DH la'Asuyei) who rules that a woman may read for a man. This implies that a woman's obligation to read the Megilah is the same as a man's.
(b) TOSFOS in Sukah (38a, DH b'Emes Amru) cites the BEHAG who says that women are unable to discharge a Tzibur (a large group) of men from its obligation to read the Megilah, because such an act involves a lack of modesty and constitutes a breach of Kavod ha'Tzibur. However, a woman is able to read the Megilah for an individual man.
(c) TOSFOS here (DH Nashim) and in Erchin (3a, DH la'Asuyei) understands the words of the Behag differently. Tosfos says that the Behag means that women may read the Megilah only for other women, but not for men.
What is the Gemara's point in teaching that women are obligated in the Mitzvah of Megilah? The Gemara's point is to teach that women have an obligation only to hear the Megilah, but not to read it. One might have thought that women have an obligation to read the Megilah, and thus they need another person -- who is also obligated to read the Megilah -- to discharge them from their obligation. The Gemara teaches that they fulfill their obligation by hearing the Megilah read by another woman. Men, on the other hand, are obligated to read the Megilah, and therefore a woman cannot fulfill the Mitzvah for a man.
What is the logic behind this difference between a woman's obligation and a man's obligation?
The MARCHESHES (ha'Ga'on Rav Henoch Eigis of Vilna, may Hash-m avenge his death) explains that the reading of the Megilah is comprised of two components: the Megilah reading serves as a Zechirah (verbal declaration of remembrance) of Mechiyas Amalek (see Megilah 18a; the Avnei Nezer infers this from a Yerushalmi), and the Megilah reading is "Pirsumei Nisa" -- it publicizes the miracle of Purim. The Marcheshes explains that women are obligated only in the component of Pirsumei Nisa, because they were included in the miracle of Purim. Women are not obligated to read the Megilah in order to fulfill the Mitzvah of Zechiras Mechiyas Amalek, because only those who are obligated to go to war against Amalek (i.e. men) are obligated in the Mitzvah of Zechiras Mechiyas Amalek (SEFER HA'CHINUCH #603). (See also OR SAME'ACH, Hilchos Megilah 1:1, who gives a similar explanation.)
HALACHAH: The SHULCHAN ARUCH (OC 689:2) first quotes the opinion that anyone who is obligated to hear the Megilah may also read it for others. He then quotes the opinion that women may not read the Megilah for men. The MAGEN AVRAHAM (689:5) writes that the second opinion is the Halachah, and this is the conclusion of the MISHNAH BERURAH (689:7). The SHA'AR HA'TZIYUN (689:15) adds that a woman also should not read the Megilah for a group of other women (see KORBAN NESANEL to Megilah 4a, 4:40). (See Insights to Erchin 3:1.)

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