1) "LAV SHE'NITAK L'ASEH" WITH REGARD TO "ONES"
QUESTION: Rava states that when a person does something which the Torah prohibits, his act is ineffective ("Iy Avid Lo Mehani"). Why, then, is the person punished, if his act does not produce any effect? Rava explains that he is punished because he acted contrary to the Torah.
The Gemara questions Rava's opinion from a Beraisa that teaches that when a Yisrael divorces the woman he raped (and was thus required to marry), he transgresses the prohibition, "He may not divorce her all of his days" (Devarim 22:29). However, he is not punished with Malkus because this prohibition is a "Lav she'Nitak l'Aseh," since there is a Mitzvas Aseh for him to remarry her. According to Rava, the man should receive Malkus for attempting to divorce the woman (even though he remarries her), because Rava maintains that even when one's act is not effective, he still is punished for transgressing the will of Hash-m.
Why does the Gemara challenge only Rava's view from the Beraisa that says that the man does not receive Malkus? The Gemara should challenge the view of Abaye from the fact that the man is required to remarry the woman that he just divorced, which shows that his transgression is ineffective (as the Gemara indeed asks in the Lishna Acharina)! (See following Insight.)
ANSWER: TOSFOS (DH Meisivei) explains that the Gemara's Lishna Kama, the first version, knew that there is a Gezeiras ha'Kasuv that requires the man to remarry the woman. Therefore, even Abaye -- who maintains that an act done in violation of the Torah normally is effective -- agrees that the divorce must be "undone" in this case. According to Rava, however, the man still should be given Malkus for attempting to transgress the Torah by divorcing her.
However, if the Gemara knew that there is a Gezeiras ha'Kasuv requiring the man to remarry the woman, then there should also be no question on Rava's opinion! The Gezeiras ha'Kasuv to remarry the woman obviously makes the prohibition to divorce her into a Lav she'Nitak l'Aseh, exempting him from Malkus.
TOSFOS explains that the Gemara assumed that the prohibition against divorcing an Anusah is not considered a Lav she'Nitak l'Aseh, but rather it is a "Lav she'Kidmo Aseh," a Lav preceded by an Aseh. Since the Aseh can be fulfilled without transgressing the Lo Sa'aseh (he must fulfill the Aseh to marry his Anusah in the first place), the Lav which prohibits divorcing her is not considered a Lav she'Nitak l'Aseh.
The Gemara concludes that although the Mitzvas Aseh to marry one's Anusah exists even before the man divorces her, the fact that the words "all of his days" (expressing the Mitzvas Aseh to remain married to her) are written in the verse after the words "he shall not divorce her" (expressing the Lav) and not after the words "he must take her for a wife" (expressing the Mitzvas Aseh to marry her) teaches that the Lav is considered a Lav she'Nitak l'Aseh.
2) DOES A PROHIBITED DIVORCE TAKE EFFECT
QUESTION: Rava states that when a person does something which the Torah prohibits, his act is ineffective ("Iy Avid Lo Mehani"). Why, then, is the person punished, if his act does not produce any effect? Rava explains that he is punished because he acted contrary to the Torah.
The Gemara questions Rava's opinion from a Beraisa that teaches that when a Yisrael divorces the woman he raped (and was thus required to marry), he transgresses the prohibition, "He may not divorce her all of his days" (Devarim 22:29). However, he is not punished with Malkus because this prohibition is a "Lav she'Nitak l'Aseh," since there is a Mitzvas Aseh for him to remarry her. According to Rava, the man should receive Malkus for attempting to divorce the woman (even though he remarries her), because Rava maintains that even when one's act is not effective, he still is punished for transgressing the will of Hash-m.
The Gemara assumes that when the man divorces his wife against the law of the Torah, the divorce is ineffective. This is difficult to understand. The fact that the man is required to remarry the woman seems to indicate that the divorce is a binding and effective act (because if it was not binding, then there would be no need for another marriage ceremony)!
ANSWERS:
(a) TOSFOS (DH Meisivei) explains that there is a Hekesh that compares divorce with the death of a husband (both of which release the woman from the marital bond). This Hekesh teaches that just as the death of the husband always has the legal consequence of changing the woman's status to that of an unmarried woman, divorce always has that legal consequence, even if the divorce involved a transgression of a Torah prohibition.
(b) The SHITAH MEKUBETZES (#24) answers that the Torah's command to the man to remarry his Anusah shows that the divorce has legal effect, even though he was prohibited from divorcing her. If the divorce has no legal effect, then he should not have to remarry her with a new marriage ceremony.

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