Mikvaos, Chapter 3, Mishnah 1. This mishnah continues the sugya of mayim she'uvim, drawn water, and asks a question that will shape the whole chapter: at what point does a mikveh actually acquire the status of being invalid, and once it has acquired that status, can it ever be undone?
To place the mishnah, we need the three rules the chachomim established regarding mayim she'uvim, water that was gathered into a vessel:
- If three lugin of drawn water fall into a mikveh before it has reached its full measure of forty se'ah of rainwater, the mikveh is invalidated.
- If less than three lugin fell in while the mikveh was still filling, and only afterward did the mikveh reach forty se'ah, the small amount of drawn water does not invalidate it. It simply does not count toward the required measure.
- Once a mikveh is already complete with forty se'ah, even a large quantity of drawn water falling in afterward does it no harm at all.
With those principles established, the mishnah cites Rebbi Yosi. Imagine shnei mikva'os, a pair of neighboring pools, she'ein bahem arba'im se'ah, and in neither of them is there the required forty se'ah, kept apart by a wall standing between them. Into the first there fell one and a half lugin of drawn water, and an identical quantity fell into the second, on the far side of the barrier: venaflu lazeh log umechtzah ulazeh log umechtzah. Later the wall gave way and the two pools ran together, nis'arvu. Suppose each side had been holding twenty se'ah. At the instant of the merger there stands one pool containing the full forty se'ah, and inside it are three lugin of drawn water.
The verdict is k'sheirim, both are fit, and the mishnah supplies the reason: mipnei shelo nikra aleihem, because there was never applied to them sheim p'sul, a title of disqualification. Review the order of events. As long as either pool stood below the forty se'ah, what had entered it was a single log plus half of another, short of the three lugin that ruin a mikveh. There was therefore no point in the history of either pool at which it could be labeled unfit. Then, when the divider collapsed, the two halves became in one instant a single gathering of forty se'ah of rainwater. The completion of the measure and the presence of three lugin of drawn water happened simultaneously, and drawn water poses no problem to a mikveh that is already whole.
The mishnah then turns the case around. Take a pool holding less than the forty se'ah into which three full lugin of drawn water descended. Right then it is ruined. Afterward the water is split in two, nechlak lishnayim, and since we work with the assumption that the drawn water spread evenly through the entire body of water, neither half now holds a whole three lugin. The halacha is nevertheless posul. Even should each half then be filled out to forty se'ah with rainwater, it stays unfit, mipnei shekara alav, because there had already been called upon it sheim p'sul: the instant three complete lugin arrived, the disqualification took hold. The water itself turned into water that cannot serve as a mikveh, and no later addition of forty se'ah can mend it.
Opposing this stands Rebbi Yehoshua machshir: Rebbi Yehoshua rules such a pool fit. The root of the machlokes is how each Tanna understands the mechanics of the p'sul. To Rebbi Yehoshua, drawn water spoils a mikveh only while a whole three lugin of it remain in place; the moment we can claim that three intact lugin are no longer there, the water is restored to fitness. To Rebbi Yosi, whose opinion the mishnah presents as the opening anonymous ruling, the arrival of three lugin has spoiled the water of the mikveh itself, and nothing can turn that back.
The mishnah preserves how far Rebbi Yehoshua pushed this. Shehayah Rebbi Yehoshua omer, it was his practice to teach that where a pool lacking the forty se'ah has taken in three lugin of drawn water, chasar afilu kortov, should it then lose even a kortov, the merest trace of water, kasher, it is fit. The reasoning: the three lugin are treated as spread through every portion of the pool, so whatever drop leaves takes along with it a fraction of them. What remains is no longer three whole lugin, and the mikveh has come back to being fit.
To this the chachomim respond, vachachomim omrim: le'olam hu bifsolo, the pool stays in its state of disqualification even once that drop has drained away. From the moment three lugin entered and spoiled the water, the chachomim pronounced it unfit, and the departure of a droplet does not dissolve their ruling.
Yet even the chachomim allow a leniency, and it flows from the nature of the halacha itself. The disqualification of three lugin is a decree of the chachomim, a din d'rabbanan, and in matters of d'rabbanan we are permitted to be toleh, to lean on a reasonable assumption that the problem is no longer present, even when that assumption is something of a stretch. So the chachomim conclude: le'olam hu bifsolo ad sheyeitzei mimenu m'loyo va'od, it stays invalid until there has flowed out of it its full measure and something more.
Here is how that works out. A mikveh short of forty se'ah received three lugin of drawn water and was disqualified. It then filled up to forty se'ah, but that entire forty se'ah is unfit, since the water itself carries the invalidity. Now fresh rainwater begins flowing in and pushing the old water out. Once forty se'ah have been displaced, we may say that the original invalid forty se'ah has been driven out, though we would still worry that three lugin of drawn water linger within. Therefore we require va'od, a bit beyond the forty se'ah. With that extra measure gone we are prepared to say that what left the mikveh was the invalid water together with at least some of the drawn water, so that three complete lugin no longer remain, and the disqualified original water is no longer present either. What stands in the mikveh now is the fresh rainwater, and it is a kosher mikveh.
So Mishnah 1 of our chapter draws the line at the moment the name of invalidity attaches. Where it never attached, as in Rebbi Yosi's two mikva'os that merged into forty se'ah in a single instant, the mikveh is fit. Where it did attach, dividing the water or losing a drop will not help according to the chachomim, and only the full process of m'loyo va'od, displacement of the entire measure and a little more, restores the mikveh to validity.