Mikvaot, Chapter Two, Mishnah Two
1) If a mikveh was measured and was found lacking [in its prescribed quantity], all things which had been purified in it, whether in private domain or in a public domain, are accounted unclean retroactively.
2) To what does this rule apply? To a serious uncleanness.
3) But in the case of a lesser uncleanness, for instance if he ate unclean foods or drank unclean liquids, or if his head and the greater part of his body entered into drawn water, or if three logs of drawn water fell on his head and the greater part of his body, and he then went down to immerse himself and he is in doubt whether he immersed himself or not, or even if he did immerse himself there is [still] a doubt whether the mikveh contained forty seahs or not, or if there were two mikvehs, one containing forty seahs and not the other, and he immersed himself in one of them but does not know in which of them he immersed himself, in such a doubt he is accounted clean.
4) Rabbi Yose considers him unclean, for Rabbi Yose says: anything which is presumed to be unclean always remains in a condition of unfitness until it is known that it has become clean; but if there is a doubt whether a person became unclean or caused uncleanness, it is to be accounted clean.
Section one: If there is a mikveh that is assumed to be large enough to purify the person or things being immersed and then it turns out that the mikveh actually did not contain enough water, then everything purified in it is retroactively accounted impure.
This is true even if this occurred in the public domain, where doubtful impurity is usually considered pure. The rule here is that the item or person immersed reverts back to its last established status, which in this case is impure.
Section two: However, this ruling, as well as the ruling in mishnah one, apply only to a type of impurity that is considered serious, for instance if he had contracted an impurity that would make him a “father of impurity.” If it was of lesser origin then he is pure.
Section three: In all of the following cases his impurity is only of rabbinic origin. The mishnah gives a couple of examples of this. The first is when a person ate or drank something unclean. The second is concerning drawn water. The rabbis decreed that drawn water defiles if a person immerses his head and most of his body in them or if three log of drawn water falls on him (we shall learn more about this later). In both of these cases, the doubt is deemed pure because the impurity is only derabanan.
The second part of this list is a repeat of the list in yesterday’s mishnah. Yesterday’s mishnah declared him impure if the source of impurity was a father of impurity. If it was of a lesser nature, then the doubtful case is ruled pure.
Section four: Rabbi Yose says that even if the impurity was a lesser one, a person or thing remains in his presumptive status until we can be sure that he or it was purified in a proper mikveh.
However, if the doubt is whether or not he ever became impure in the first place, or whether he ever defiled something else, for instance we don’t know whether he ate unclean food, or we know that he ate unclean food but we don’t know if he touched something else, then the doubt is ruled pure. This is because he, or the thing he might have defiled, did not have a presumptive status of being impure.
In sum, the sages and R. Yose disagree as to what principle we invoke in these types of situations. According to the first opinion, if the impurity is serious we rule stringently and if it is lesser than we rule leniently. Rabbi Yose says that the principle is that a person retains his presumptive status.