We are beginning the text of siman 7. The Chayei Adam writes that an achbar (which can translate to rat or mouse) is considered b’mino nitzad, because there is a type of achbar which was used for its skin. The Chayei Adam continues, and writes that even though it is not used for its skin where he lives, since it is known that there are places where other species of achbar are used for their skin, it is considered b’mino nitzad. As we learned yesterday, other poskim disagree on this point, and consider an achbar ain b’mino nitzad, given that such a minimal amount of the species is used.
However, even if we assume it is b’mino nitzad, the next question would be why the person is trapping it. If it is being trapped to use the achbar itself, it would be a melacha she’tzricha legufa, and it would be assur mideoraysa. If he plans to trap it to discard it, it is a melacha she’eina tzricha legufa, the Chayei Adam writes that melacha she’eina tzricha legufa is chayav. However, according to Tosfos, melacha she’eina tzricha legufa is assur miderabanan. According to the Rambam, that melacha she’eina tzricha legufa is assur mideoraysa, it is completely assur to set up a trap on Shabbos. If it immediately traps an achbar it would be assur mideoraysa, so it is assur miderabanan to set it up. It would be muttar to set it up before Shabbos.
However, many poskim do not follow the Rambam, but instead pasken like Tosfos that melacha she’eina tzricha legufa is patur. Thus, even if we accept that an achbar is b’mino nitzad, it would only be assur miderabanan because it is a melacha she’eina tzricha legufa. Certainly, if one argues it is ain b’mino nitzad, they are both issurim derabanan.
In regards to making the trap itself, the Chayei Adam does not directly address the point, but adds in parentheses that the Magen Avraham writes that it is assur to set up traps on Shabbos. The Chayei Adam argues that the Magen Avraham should have added that this only a concern according to the Rambam, who holds there is a true concern for an issur deoraysa. If there is no issur deoraysa, there is no gezeira derabanan against setting up the trap on Shabbos itself. The Mishnah Berurah quotes the Magen Avraham, but does not add the qualification of the Chayei Adam, which would seem to imply that the Mishnah Berurah paskens in accordance with the Rambam that there is potential for an issur deoraysa when trapping the achbar. Elsewhere, the Mishnah Berurah paskens against this Rambam, so it is difficult to understand why the Mishnah Berurah brings it down in this manner.
Summary
The Chayei Adam paskens that mice are considered b’mino nitzad, and even if a person in a specific scenario does not want the mouse, such that it is a melacha she’eina tzricha legufa, he is concerned for the Rambam that melacha she’eina tzricha legufa is chayav. The Mishnah Berurah seems to pasken for this concern, which is in accordance with the Rambam, even though elsewhere he is clear that most rishonim argue on the Rambam, and their opinion is normative halacha.
Since there is a concern for an issur deoraysa, it is assur to set up a trap on Shabbos as well. It would be muttar to set up the trap before Shabbos.
