Mikvaot, Chapter Two, Mishnah Three
Drawn water that falls into a mikveh that has less than forty seahs of water is invalid and remains invalid even if more kosher water comes into the mikveh. In Toharot 4:7 we learned that if there is a doubt about whether drawn water went into the mikveh, the rabbis declared it clean. Our mishnah deals with what sort of case of doubt is pure and what case of doubt must be ruled impure.
The case of a doubt about drawn water which the sages have declared clean;
1) If there is a doubt whether [three logs of drawn water] fell into the mikveh or not,
2) Or even if they did fall in, there is a doubt whether [the mikveh] contained forty seahs or not,
3) Or if there were two mikvehs one of which contained forty seahs and the other did not, and drawn water fell into one of them and it is not known into which of them it fell,
4) In such a doubt it is accounted clean, because there exists [a possibility] on which we may depend [in declaring it clean].
5) If they both contained less than forty seahs, and [drawn water] fell into one of them and it is not known into which of them it fell, in such a doubt it is accounted unclean, because there exists no [possibility] on which we may depend [in declaring it clean].
Section one: The first possible doubt is whether or not the three logs of drawn water fell into the mikveh in the first place.
Section two: If the mikveh contained forty seahs of valid water (not drawn) then even if three logs of drawn water fall in, the mikveh remains kosher. So the second level of doubt is whether or not the mikveh was large enough to nullify the three logs.
Section three: In this case there are two mikvaot, one of which is large enough to nullify the three logs and we are not sure into which mikveh the logs fell.
Section four: In all of these cases, we can consider the mikveh clean. If the mikveh currently has less than 40 seahs, we can add more water. The reason that the doubt is considered clean is that there is a possibility that the mikveh or both mikvaot are valid.
Section five: In contrast, in this case one of the mikvaot is definitely invalid because neither had forty seahs. Therefore, we have to treat both mikvaot as if they are invalid.