The Mishna in Masekhet Pesachim (9a) establishes that after a person searched one area of his home for chametz before Pesach, he does not have to be concerned that perhaps a rodent afterward brought chametz into that area. If Halakha had required us to be concerned about such a possibility, the Mishna explains, then “ein la-davar sof” – “there is no end.” Meaning, one would never be able to satisfy the obligation to ensure the absence of chametz, as he would always have to entertain the possibility that a rodent brought chametz somewhere in his home. Thus, although such a possibility exists, Halakha cannot expect a person to go this far in ensuring the absence of chametz from his property.
Tosefot raise the question of why this halakha needed to be mentioned. After all, the previous Mishna (2a) already stated that one is not required to search for chametz in a part of the home where people do not normally bring chametz. Of course, even if people do not normally bring chametz to certain areas of the home, it is possible that rodents carry chametz there. Therefore, once the Mishna established that such places do not require checking before Pesach, we may conclude that Halakha does not require us to concern ourselves with the possibility of chametz brought into the home by rodents. Accordingly, Tosefot ask, why must the next Mishna need to instruct that an area may be considered free of chametz after it is checked, despite the risk of animals bringing chametz to that area?
Tosefot suggest that one might have understood the first Mishna as referring specifically to places where rodents are uncommon. If not for the second Mishna, one could have figured that the Mishna does not speak at all of the risk of animals bringing chametz, and addresses only the case of a home that is presumed free of rodents. In such homes, areas where people do not bring chametz do not require searching before Pesach. However, in homes where rodents indeed scurry about, and may carry pieces of food with them, one might have assumed that such places require checking even if chametz is not usually brought there, and even after it had been searched.
A number of writers noted that the Rambam, in his codification of this halakha, avoids Tosefot’s question. In Hilkhot Chametz U-matza (2:7), the Rambam codifies these two Mishnayot together in a single passage. Meaning, he understood that the second Mishna, which rules that one does not have to be concerned about chametz being brought by a rodent, refers not to a place which had been searched for chametz, but rather to a place that is not generally used for chametz. The second Mishna’s intent is precisely to clarify that the first Mishna applies even if rodents are present and could conceivably bring chametz into areas where people do not bring chametz. Despite this possibility, Halakha does not require searching in such areas, as we would then never be able to declare any area free of chametz.