Masechet Mikvaot begins by listing the various natural collections of water and how they differ with the respect to taharah. The first one mentioned is the mei gevaim, which we discussed in detail previously (Mei Gevaim, 6(50)). The first Mishnah teaches that if someone tameh drinks from the water, someone tahor should not drink from that water otherwise they will become tameh. One explanation we saw previously, was that even though the mei gevaim are not susceptible to tumah until they are intentionally extracted, we are concerned that tameh drops of water fell back into the water. When the tahor person takes some water to drink, it becomes susceptible to tumah. The concern is that the water will include the tameh drops which would then make that water tameh. If he drinks the tameh water, he would become a sheni le'tumah (by way a rabbinic decree).
The Mishnah (1:4) teaches that during the dry months, those mei gevaim that were accessible for drinking water are assumed to be tameh. The ruling is based on the concern above, that someone tameh may have drank from the water.
The next Mishnah records a debate regarding when the mei gevaim can be assumed to be tahor again. Beit Shammai maintain that it is when the rainwater collects and exceeds the water that was already there, and the water also overflows. Beit Hillel understand that spilling over is not necessary. Finally, R' Shimon takes the most lenient position, that even if a small amount of rain water collects, yet it spills over, the mei gevaim would become tahor.
The Mishnah Achrona asks why Beit Shammai also needs the water to spill over. Why is it not sufficient that the existing water become annulled in a majority of new tahor water? He also questions the position of R' Shimon. What does water spilling over achieve?
The Mishnah Achrona bases his answer on a later Mishnah (3:3). The Mishnah teaches that if a mikveh was filled with drawn water, and there is stream of water passing through it, then the mikveh is only valid once we can be certain that none of the original water remains. The Raavad there explains that if the mikveh was invalid since it had three log of drawn water, since that pesul is rabbinic, once three log of water of the stream passes through we can assume it pushed out the invalid three log. R' Shimon therefore considers that this case should share the same law, since biblically, that drop of tameh water should already be annulled in the majority.1
Both Beit Shammai and Beit Hillel however require the existing water be annulled in the majority of rainwater. For Beit Shammai however, that alone is insufficient. The Mishnah Achrona explains that they are concerned that someone might wait and anticipate the rainfall to purify their mei gevaim. The issue then would be that since one wanted that rain, it would become susceptible to tumah. Not only could it not purify the mei gevaim, but it would become tameh. Consequently, an additional requirement was there either as a heker (a reminder) that there can be an issue of machshava or to ensure rabbinic counsel is sought.
One might ask, why is a majority of fresh water necessary. The drops of tameh water in the mei gevaim are already in the minority.2 The Mishnat Taharot cites the Chazon Ish (kama 1:5) that explains that the original tumah of the water in the ground is indeed rabbinic.3 If there was no avenue for making it tahor it would never become tahor. Beit Shammai and Beit Hillel each provide that way.
1 This explanation fits those Rishonim that explain, as we did in the beginning of the article, that the issue the mei gevaim is a tameh drop. Recall, that according to the Rambam however, the mei gevaim became tameh. This is because the Rambam understands that mei gevaim can become tameh if the tumah is brought into contact with the mei gevaim willingly. We would need to understand how the water spilling over alone would help according to the Rambam.
2 According to the Rambam (cited in the previous footnote) that all the mei gevaim are tameh, the need for a majority of fresh water is readily understood.
3 The Mishnat Taharot explains that this could either be since tameh liquids ability to make other liquid tameh is rabbinic or because the logic that the single tameh drop will be "awakened" (chozer ve'neiur) and make the other water tameh is rabbinic.
Receive our publication with an in depth article and revision questions.
Listen to the new Mishnah Shiurim by Yisrael Bankier