QUESTION: Rebbi Yochanan states that if one inadvertently has relations with his wife when she is a Nidah, he is obligated to bring a Chatas. If one inadvertently has relations with his Yevamah (the wife of his deceased brother who left behind no living offspring after his death) who is a Nidah, he is exempt, because he is "To'eh b'Devar Mitzvah."

The Gemara gives two explanations for why one is liable when he has relations with his wife who is a Nidah. The first explanation is that he did not accomplish any Mitzvah with his act, and one who is "To'eh b'Devar Mitzvah v'Lo Asah Mitzvah" is not exempt. The second explanation is that his mistake in this case was avoidable, because he should have asked his wife if she was a Nidah. His act is considered a Shogeg and not a mistake ("To'eh") and he must bring a Chatas.

The Gemara asks that he still should be exempt in this case, because his act fulfills the Mitzvah to procreate (Piryah v'Rivyah), or the Mitzvah of Onah, and thus he should be considered a "To'eh b'Devar Mitzvah v'Asah Mitzvah."

What is the Gemara's question? The Gemara gives two explanations for Rebbi Yochanan's statement, and this question is not appropriate according to either one. When a man has relations with his wife who is a Nidah, he fulfills neither the Mitzvah of Piryah v'Rivyah nor the Mitzvah of Onah. Why, then, is this case considered a case of "Asah Mitzvah"? The man's act fulfills no Mitzvah, and thus he certainly should be liable.

According to the second explanation for Rebbi Yochanan's statement, it is obvious that his act cannot be considered a mistake, because he should have asked his wife if she was a Nidah. Why, then, does the Gemara ask that he should be exempt because of "To'eh b'Devar Mitzvah"? (TOSFOS DH b'Ishto)

ANSWER: TOSFOS explains that the Gemara's question applies to the second explanation for Rebbi Yochanan's statement. According to that explanation, even one who is "To'eh b'Devar Mitzvah v'Lo Asah Mitzvah" is exempt. Accordingly, one who has relations with his wife when he is unaware that she is a Nidah and he thinks that his act fulfills a Mitzvah should be exempt, even though he does not actually fulfill any Mitzvah. The Gemara therefore asks why he is liable if he intended to do a Mitzvah.

Why, though, should he be exempt, if the Gemara has already explained that he is not considered a "To'eh b'Devar Mitzvah" because he should have asked his wife if she was a Nidah? The TOSFOS SHANTZ explains that when the Gemara says that he should have asked his wife, it already knows the answer that will be expressed later -- that he should have asked her whether she was Samuch l'Vestah or a Nidah. The Gemara here is merely explaining how it arrives at that conclusion, and in doing so it discusses all of the other options and explains why they are not viable (that is, it cannot be that she is pregnant, because there is still a Mitzvah of Onah, and so on).

(It is not clear whether RASHI agrees with this reasoning. Rashi (DH Ishto Nidah) seems to understand that the Gemara's question is even according to the first explanation for Rebbi Yochanan's statement. Rashi prefaces both explanations for Rebbi Yochanan's statement by pointing out that the reason why the case in question is not considered a case of "To'eh b'Devar Mitzvah" is because "she was pregnant and Samuch l'Vestah.")