See these two posts:
I referred to these posts in the course of a chevrusa today. And I found a comment on the latter one that I did not notice previously:
AnonymousSunday, June 30, 2013 10:33:00 AM
Hi, I have used this understanding of Kavua before, it is quite elegant. However how does it fit with the Kavua of the mouse in Pesachim which just came up in Daf Yomi?
This sugya is cited by Prof. Moshe Koppel (cited, in passing, in the second blog post above) in his essay at http://etzion.org.il/en/principle-kavua:
The gemara (9b) discusses a scenario where nine piles of matza and one pile of chametz are lying around before Pesach. A mouse comes and takes a piece from one of the piles and enters a house. However, we do not know if it took chametz or matza and thus are in doubt whether the house must be checked once again for chametz. The gemara distinguishes between a case where the mouse is seen taking a piece directly from one of the piles ["kavu'a"] and an instance where the piece snatched by the mouse was first isolated from the piles ["parish"]. These two cases are said to be analogous, respectively, to two cases considered in Ketubot (15a): "If there are nine stores which sell kosher meat and one which sells non-kosher meat and someone took [meat] from one of them but he doesn't know from which one he took, the meat is forbidden. But if [a piece of meat] is found [not in a store], follow the majority." Thus if the majority of stores from which the meat might have originated are kosher, the meat is permitted.
The question is a good question: Where is the "moral hazard" in the case of the mouse?
I think the answer may be found in a word in the Rambam (Chametz u'Matza 2:10). The Rambam does not understand the case as one in which piles are just "lying around." He begins the halacha with the word heini'ach - he placed.
I would suggest that it is the involvement of the owner of the house that places him in a situation of "moral hazard."
There is another possibility, not as elegant, that this case is different because the owner will be compelled (or not) to redo Bedikas Chametz. Accordingly, his "negi'ah" is his "moral hazard."