Masechet Pesachim open with the laws regarding checking for chametz. The second Mishnah teaches that after checking one room, we need not be concerned that a chulda (weasel) dragged some chametz from elsewhere into the room that was just checked. Similarly, one need not be concerned that after cleaning their house that a chulda brough chametz from another house. The Mishnah explains that this is because if we were concerned then ein ledavar sof - "there would be no end". In other words, if we were concerned that chametz may be dragged from place to place, then checking for chametz would be impossible. The Tifferet Yisrael explains that one could ally the concern that chametz was dragged from one place to another within the house, by coordinating a team of people to check all the rooms at the same time. Nevertheless, being concerned for that scenario would mean that one would need to be concern that perhaps chametz was taken from house to house, or chatzer to chatzer. Coordinating that all houses are checked at the same time would be impossible.
The Sfat Emet (Eiruvin 9a) finds the Mishnah difficult. The Mishnah implies, that if there was "an end", then one would need to be concerned that the room one checked, might later have chametz in it. In other words, without ein ledavar sof, one would need to rule stringently and require rechecking unless all the rooms were checked simultaneously. If that is that case, why is ein ledavar sof a valid justification for checking one room at a time enough? The Sefat Emet also asks, that on a biblical level, bitul, annulling all the chametz in one's possession, is enough. The requirement to check is rabbinic. That being the case whether the chulda dragged in chametz is a doubt regarding a rabbinic law for which one can rule leniently even without ein ledavar sof.
The Sefat Emet answers the indeed bitul would be sufficient on a biblical level. The Chachamim however required bitul, either because they were concerned one would not do a wholehearted bitul (Ran) or in case one finds a nice portion of chametz on Pesach and eats it by mistake (Tosfot). The Sefat Emet explains that ein ledavar sof meant that had we been concerned for the chulda, then it would undermine this rabbinic law; it would be impossible to implement. Consequently, it is not due to a doubt related to rabbinic law, but rather it was built it to the rabbinic law from the outset that one does not need to be concerned for the involvement of the chulda.
The Sefat Emet continues that this explanation answers the question posed by the Tosfot. The Tosfot ask that the ruling in our Mishnah appears unnecessary. The first Mishnah already taught that one does not need to check rooms into which chametz is not ordinarily brought. The Mishnah already implies that one need not be concerned that chulda dragged chametz into those location. Consequently, we already know we do not need to be concerned for a chulda taking chametz from place to place.
The Sefat Emet answers that the reason behind each Mishnah is different. The reason behind the first Mishnah is because the Chachamim were not stringent to require a person to check in this case of a safek (doubt). In our Mishnah however, the reason why one need not be concerned for the chulda was because it was not built into the original gezeira. Furthermore, based on the first Mishnah alone one might think that you need to a team of people to check all rooms at the same time considering it is a location in to which chametz was brought. In other words, the first Mishnah simply teaches that bedika is not required in places where chametz is not brought. In a house however, that already has the obligation to check for chametz, one might have thought that one needs to be concerned that a chulda might ruin the inspection. Consequently, our Mishnah teaches that that concern was not build into the original gezeira.
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