Mikvaot, Chapter Two, Mishnah One
Today’s mishnah deals with a case of an impure person who immersed himself in a mikveh in order to become pure, but it is unclear whether he did so in a manner that would purify him. In all such cases he is not considered clean. The reason is that his last determinable status was unclean. Therefore, he remains unclean until we can be sure that he is actually clean.
The mishnah is pretty straightforward, so there is no explanation below.
1) An unclean man who went down to immerse himself:
2) If it is doubtful whether he did immerse himself or not;
3) And even if he did immerse himself, it is doubtful whether the mikveh contained forty seahs or not;
4) And if there were two mikvehs, one containing forty seahs but the other not containing forty seahs, and he immersed himself in one of them but he does not know in which of them he immersed himself,
5) In such a doubt he is unclean.