The Mishnah (2:3) discusses the status of a mixture of rainwater and mei shefichut (waste water). Mei shefichut is assumed to be tameh, therefore it appears we are dealing with a mixture of tameh and tahor water. The Mishnah initially teaches that it depends on which water constitutes the majority. If however they are equal, we rule stringently and the mixture is tameh. The Mishnah then continues by explaining that the proportions are only the determining factor if the mei shefichut was first in the mixture. If however it was the rainwater, then even if only a small amount of mei shefichut was poured in, the mixture is tameh. We shall try to understand why.
The Rambam (Tumat Ochlin 2:26) explains that when the rainwater precedes the mei shefichut, then the latter causes the former to become tameh. The Raavad comments that he is unsure as to the reason why the order is important. He suggests that perhaps it is because since the rainwater fell first and was long forgotten it too takes on the status of mei shefichut. The Kesef Mishnah adds that one might ask that if that were the case, the rainwater should be tameh even without the additional mei shefichut. He answers that the gezeira of mei shefichut, giving it a chazaka of tumah, is only when it has a shem (name) of mei shefichut.
The Kesef Mishnah however has a problem with the Rambam. If the rainwater preceded the mei shefichut we should say that each drop of the latter should be annulled in the majority of the water that is there. He therefore suggest that we are dealing with a case where the water is not still. Consequently, the status of the mixture is determined by whatever came last.
The Chiddushei HaRosh Troib, however explains that when it comes to tumah and tahara the concept of bitul (ordinarily) does not apply. With respect to liquids however, if they were not initially le’ratzon, then since they are not susceptible to tumah, bitul can apply. When the rainwater is first and resting there before us, the owner already considers them and they can become tameh irrespective of the quantity of mei shefichut it mixes with. If however the rain water follows the mei shefichut, since they have yet been considered, they are not susceptible to tumah and can mevatel the mei shefichut.
The Tosfot Chadashim explains in a similar manner, that we are dealing with a case where the water collects in a kli. If the mei shefichut is in the kli first, the owner clearly is not interested in the rainwater that falls later since it is falling in dirty water. It is not, like the previous explanation, that he has yet to consider the rain water, but rather he will not consider it. If, however the rainwater falls first and the water is useful, even a small amount of tameh liquid would cause it to become tameh.
The Mishnah Achrona goes ones step further. He assumes that we are dealing with a kli or pit that was set aside to fill with drinking water. He explains that since he is anticipating the rainfall, if it precedes the mei shefichut then clearly he is machshiv the water and it is susceptible to tumah. If it first contains the mei shefichut, the rainfall will be of no benefit as it will immediately be spoiled and therefore not susceptible to tumah.
The Mishnah Achrona finds support for this line of reasoning from another Mishnah (4:10). There it teaches that if tameh water falls on wood followed by a greater quantity of rainwater, then it is tahor. If however one took the wood outside for them to rained upon, then they are tameh. He notes that in that case, the Mishnah does not differentiate between the order in which the water falls, but instead whether one took the wood outside to be rained upon. He explains that in that case, the order is irrelevant because one does not want the wood to get wet. It is only when one demonstrates that he does – when he takes them outside to be rained upon – that the water would become tameh.
The Chazon Nachom explains that we find that the laws of hechsher and tumah are dependent on the ratzon of the owner. When a person pours the mei shefichut on the rainwater it is clear that he wants to make the collection a pool of waste water; so the entire mixture is tameh. If however the mei shefichut was first, then it is not obvious that he wants it to be all waste water. Instead we assume the he is happier to purify the mei shefichut and be annulled in the majority rain water that falls.1
1 Rav Lichtenstein (Shiurei Haral 128) cites the Rid maintains that there is not bitul at all when it comes to tumah. Our case of Mei Shefichut however is a special gezeira in that it is assumed to be tameh, consequently it is only tameh when it is be’ein, ע"ש.
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