Toharot, Chapter Four, Mishnah Four
1) An olive’s bulk of corpse was held in a raven’s mouth and it is doubtful whether it overshadowed a person or vessels in a private domain:
a) The person’s condition of doubt is deemed to be unclean
b) But the vessels condition of doubt is deemed clean.
2) One who drew water in ten buckets and a dead sheretz was found in one of them, it alone is deemed unclean but all the others remain clean.
3) If one poured out from one vessel into another and a dead sheretz was found in the lower vessel, the upper one remains clean.
Section one: There are two reasons why the person is unclean. First of all, a person has the ability to be asked (see mishnah 3:6) and any time someone can be asked, the doubtful case is deemed impure. Second, although the impurity is moving, there is a special rule with regard to impurity transmitted through overshadowing (ohel)it defiles in cases of doubt even though it is moving.
The vessels remain pure because they don’t have intelligence such that they can be asked.
Section two: The dead sheretz found in one bucket clearly defiles that bucket. We might have thought that the other buckets should also be impure because they may have had contact with the dead sheretz while it was in the cistern. The mishnah teaches that the other buckets are pure for if we know that impurity is in one place (the bucket in which it is found) we do not assume that it might have been in another place. Furthermore, the sheretz doesn’t defile the water, for water that is still connected to its source in the ground (the well or cistern) is not susceptible to impurity.
Section three: If one is pouring from one vessel to another and a dead sheretz is found in the lower vessel, only the lower vessel is impure. We don’t assume that the sheretz was originally in the upper vessel, for the same reason that we said in section twoimpurity found in one place is not assumed to have been in another.