Mikvaos 2:1-2

Mikvaos 2:1

Let’s say that a ritually-unclean person went to immerse himself in a mikvah and there’s a doubt as to whether or not he actually immersed. Even if he did immerse, there’s a doubt as to whether or not the mikvah contains 40 seah. Or there were two mikvahs, only one of which contained 40 seah, and he immersed but he doesn’t know in which mikvah. In all of these cases, the doubt is ruled ritually unclean.

Mikvaos 2:2

If a mikvah was measured and found to contain less than 40 seah, everything that had been immersed in it, in either a public or a private domain, is retroactively ruled unclean. This only applies to the greater forms of ritual impurity, not to the lesser forms. For example, if a person ate ritually-unclean food or drank ritually-unclean liquid, or if his head and the major part of his body entered drawn water (in between immersing and nightfall), or if three log of drawn water (about 42 ounces) fell on his head and the major part of his body and he then went to immerse and there’s a doubt as to whether or not he immersed, or if he did immerse but there’s a doubt as to whether or not the mikvah contains 40 seah, or if there were two mikvahs, only one containing 40 seah, and he immersed in one of them but he doesn’t know which – in such cases as these, the doubt is ruled ritually clean. Rabbi Yosi rules such a person unclean because, in his opinion, anything presumed to be ritually unclean remains in an unfit state until it is known to have been purified. However, if there’s a doubt as to whether or not a person was rendered unclean or conveyed ritual impurity, then the case of doubt is ruled clean.