Biblically, all one has to do to prepare for Pesach is to renounce one’s chametz. Rabbinically, however, one must search for it and remove it from one’s residence so that no chametz will be found there. Once the search is completed, the chametz must be burned and renounced with intention. But then, what if one sees something that suggests that the purity of the residence has been compromised? The gemara likens it to the case of one who walks among fields where a corpse has been buried at a time when the markers of the graves have been lost. Since both situations are regulated only by rabbinic decree, there is a potential leniency if the potential contamination remains only hypothetical.
Daf 333 (Pesachim 10a-10b)
A mouse pauses by a pile of chametz and perhaps picks up some of it. The gemara seeks to determine under what circumstances a house proximate to the chametz that has already been searched must be searched again. (And if it is searched again and some chametz is found but one cannot be certain it is the chametz that the mouse dragged in, then what?) There are many analogous cases involving walking in fields that may contain graves.
When is a new search required? If a mouse is seen both entering and leaving a house with a loaf of bread, can one be sure that it is the same mouse (or the same loaf of bread)? What if the mouse who enters is white and the mouse who exits is black; is it likely the same loaf snatched from one mouse by the other? What if a weasel is seen exiting the house with a loaf after the mouse is seen entering with a loaf? Why isn’t the mouse in the weasel’s mouth as well as the loaf? And what if the mouse is also in the weasel’s mouth?
Mishnah. Here are rules for conducting a search for chametz past the appointed time, as well as for storing chametz put aside to eat between the conclusion of the search and the onset of Pesach.
Gemara. The gemara seeks to determine why R’Yehudah teaches that three searches are required.