Mikvaos, Chapter 2, Mishnah 3. Before we can follow this mishnah, one foundational halacha in the construction of a mikveh has to be clear. The chachomim ruled that drawn water, mayim she'uvim, spoils a mikveh under certain conditions: should three lugin of such water enter a pool that has not yet accumulated its required forty se'ah of rainwater, that pool is passul. What makes water she'uvim is that it was gathered up in a vessel, hence the name drawn water. The critical amount is three lugin, and the critical timing is before the forty se'ah of rainwater have come together. Our mishnah asks when this psul is imposed and in which doubtful situations the chachomim were prepared to rule leniently.
Everything here rests on the fact that invalidating a mikveh through three lugin is not Torah law but an enactment of the chachomim. Maseches Taharos, chapter 4, mishnah 7, says so in so many words: the psul of drawn water has rabbinic authority behind it, not biblical. Precisely because its status is rabbinic, the familiar principle that uncertainty in a rabbinic prohibition is decided toward leniency becomes available to us here.
The mishnah opens: "Safek mayim she'uvim shetiharu chachomim," the cases of doubt regarding drawn water in which the chachomim ruled that the mikveh remains tahor. Three such cases are listed.
- "Safek naflu safek lo naflu": we have no idea whether those three lugin ever reached the water. It is possible they went in; it is equally possible they never did.
- "Afilu naflu": here the entry itself is not in question, yet a different uncertainty remains, whether at that moment the pool already held its full complement or not. Did it have "arba'im se'ah" of rainwater in it, in which case nothing was spoiled, or was the measure still incomplete, which would render it passul? That question has no answer available to us.
- "Shnei mikvaos, echad yesh bo arba'im se'ah v'echad ein bo": two pools stand before us, and it is established that one contains forty se'ah while the second is still short of the measure. "Nafal l'echad mehen v'eino yodei'a l'eizeh mehen nafal." That the drawn water landed in one of the two is certain; which of them received it is what escapes us.
In all three of these cases the ruling is "sfeikan tahor," the doubt is resolved leniently and the mikvaos remain fit. The mishnah gives the reason: "mipnei sheyesh lo b'mah yisleh," because there is something upon which to hang the doubt. In a rabbinic doubt we are always permitted to attribute the event to the possibility that leaves no disqualification. In the last case, we say that the three lugin landed in the mikveh that had already gathered its forty se'ah, where drawn water does no harm at all.
Against these stands the case in which the mishnah rules strictly. "Hayu shneihem pechusim mei'arba'im se'ah": neither pool has yet reached forty se'ah, "v'nafal l'echad mehen v'eino yodei'a l'eizeh mehen nafal," and the drawn water went into one of the two without our being able to identify which. The verdict is "sfeikan tamei." The uncertainty is still an uncertainty in rabbinic law, yet leniency is impossible here, since it is beyond question that one of these two pools has been spoiled, and no harmless outcome exists to which the incident could be assigned. The mishnah's own explanation is "she'ein lo b'mah yisleh," the doubt has nothing to be hung upon. With no reason to single out one pool rather than the other, both must be treated as passul.
The lesson of the mishnah, then, is not simply that a rabbinic doubt is treated leniently, but that the leniency needs somewhere to rest. Where a harmless explanation is available, the chachomim allow us to rely on it. Where every possibility leads to a disqualification, the leniency has nothing to stand on.