1) DOES "MAYIM SHE'UVIM" DISQUALIFY A MIKVAH MID'ORAISA OR MID'RABANAN?
OPINIONS: Rebbi Eliezer says that a board used by bakers which was later attached to a wall is Tahor. The Gemara asks whether Rebbi Eliezer agrees with the law in the Beraisa (65b) which discusses the status of a pipe attached to a Mikvah. The Beraisa states that only if the pipe was fashioned into a pipe before it was attached to the Mikvah does it render the Mikvah invalid through the Pesul of Mayim She'uvim (water drawn by a vessel). If the pipe was fashioned into a pipe only after it was attached to the Mikvah, it does not have the status of a vessel and water that passes through it is not considered Mayim She'uvim. Does Rebbi Eliezer, who says that the baker's board can be attached to the wall and still be Tahor, argue with the Beraisa?
The Gemara explains that Rebbi Eliezer agrees with the Beraisa. The reason why he says that that the baker's board is Tahor is that it can only become Tamei mid'Rabanan at most, because it is "Peshutei Kli Etz," a wooden vessel with no receptacle ("Beis Kibul"), which cannot become Tamei mid'Oraisa. Rebbi Eliezer agrees that a pipe, which can become Tamei mid'Oraisa, renders water drawn with it Mayim She'uvim which can invalidate a Mikvah.
The Gemara asks that this implies that Mayim She'uvim involves a Halachah mid'Oraisa, but it is known that the Pesul of Mayim She'uvim is only mid'Rabanan.
Is the Pesul of Mayim She'uvim indeed mid'Rabanan, as the Gemara assumes?
(a) TOSFOS (66a, DH Michlal) quotes the RASHBAM and RABEINU TAM who maintain that Mayim She'uvim can invalidate a Mikvah mid'Oraisa. They quote the Toras Kohanim which derives from a verse that a person may not use a Mikvah filled with Mayim She'uvim. Rabeinu Tam cites further proof from the Mishnah in Mikva'os (2:4) which says that if the first Revi'is of water in a Mikvah is a Revi'is of Mayim She'uvim, the Mikvah is invalid. However, if the first Revi'is of water is valid water for a Mikvah, it requires three Lugin to invalidate it. The Mishnah implies that the first Halachah is mid'Oraisa, and the second Halachah is mid'Rabanan. (See MAHARSHA who explains how Rabeinu Tam and the Rashbam understand the Gemara here.)
However, Tosfos quotes the RI who refutes these proofs. The Toras Kohanim quotes the verse only as an Asmachta. The Mishnah in Mikva'os may mean that since a Revi'is of water is enough for the immersion of small vessels (such as needles), the Rabanan considered it an important amount of water because it can be used as a Mikvah for small vessels. This is why they enacted that a Mikvah which starts with this amount of Mayim She'uvim is invalid mid'Rabanan.
(b) The Ri, as quoted by Tosfos, maintains that the concept of Mayim She'uvim is almost entirely mid'Rabanan (with the exception of immersing in Mayim She'uvim placed in containers). This means that if a person fills up a pit in the ground (and not a vessel) with Mayim She'uvim, water drawn with vessels, it is a valid Mikvah mid'Oraisa (and it is Pasul only mid'Rabanan). The RASHBA supports this explanation by quoting the Gemara in Yevamos (82b) which states that a Mikvah which contains forty Se'ah of valid water (the minimum amount necessary for a valid Mikvah) no longer can become Pasul, unless one takes out a Se'ah of water and replaces it with a Se'ah of Mayim She'uvim, and he does this until most of the water of the Mikvah is Mayim She'uvim. The Mikvah water becomes Batel b'Rov and thus the Mikvah becomes Pasul. (The Rashba maintains that this is not an ordinary case of Batel b'Rov, but rather a "Mar'is ha'Ayin" form of Batel b'Rov; that is, it looks like there is no longer a majority of valid Mikvah water.) (Y. MONTROSE)
2) WHY "MAYIM SHE'UVIM" INVALIDATES A MIKVAH
OPINIONS: Rebbi Eliezer says that a board used by bakers which was later attached to a wall is Tahor. The Gemara asks whether Rebbi Eliezer agrees with the law in the Beraisa (65b) which discusses the status of a pipe attached to a Mikvah. The Gemara concludes that the Beraisa follows the view of the Chachamim who argue with Rebbi Eliezer and maintain that a baker's board is Tamei. If the Beraisa follows the view of the Chachamim, then why does it say that if the pipe was fashioned into a pipe only after it was affixed to the ground, the water that flows through it does not invalidate the Mikvah? The Gemara answers that Mayim She'uvim is only a Pesul mid'Rabanan.
If the Pesul of Mayim She'uvim is only mid'Rabanan, then why do the Chachamim rule that water that comes into a Mikvah through a pipe which was fashioned into a pipe and then affixed to the Mikvah invalidates the Mikvah? If the Pesul of Mayim She'uvim is only mid'Rabanan, they should be lenient in such a case. The Gemara answers that the Chachamim maintain that if something was made into a vessel and only afterward affixed to the ground, it retains its status of a vessel.
What is the Gemara's main point? Is the Gemara's intention to teach that the Chachamim maintain that the Pesul of Mayim She'uvim is mid'Rabanan, or is the Gemara's intention to teach that the Chachamim maintain that a vessel fashioned before it was affixed to the ground remains Tamei?
(a) The RASHBAM (DH Iy Hachi) explains that the Gemara's intention is to teach that the Chachamim maintain "Kol ha'Mechubar l'Karka Eino k'Karka" -- "anything attached to the ground is not like the ground." However, they do not apply this law to Mayim She'uvim since it is only a Pesul mid'Rabanan. The Gemara asks that according to the Chachamim, even something which was attached to the ground after it was formed into a pipe should pose no problem of Mayim She'uvim. The Gemara answers that such a vessel is not considered part of the ground, but rather a vessel, even after it is attached to the ground.
The TORAS CHAIM asks that this does not seem to be the flow of the Gemara. The Gemara makes a point of saying that the Pesul of Mayim She'uvim must be mid'Rabanan, and it implies that this is relevant to the opinion of the Chachamim. Why, though, does it make a difference whether or not the Pesul is mid'Oraisa or mid'Rabanan? Even if the Chachamim maintain that the Pesul of Mayim She'uvim is mid'Oraisa, they still may rule that a pipe that was fashioned into a vessel before it was attached to the ground is always considered a vessel, while something that was first attached to the ground and only afterwards fashioned into a vessel is not a vessel. This explains both the Beraisa about the baker's board and the Mikvah, and there is no need to discuss whether the Pesul of Mayim She'uvim is mid'Rabanan or mid'Oraisa. Why does the Gemara discuss this point at all?
(b) The TORAS CHAIM therefore gives a different explanation for the Gemara. He explains that when the Gemara says that the Pesul of Mayim She'uvim is mid'Rabanan, it means that even an object that was attached to the ground and only afterwards made into a vessel retains the Halachic status of a vessel. However, Mayim She'uvim that was transported through such a vessel invalidates a Mikvah only due to a decree of the Rabanan. The Rabanan decreed that three Lugin of Mayim She'uvim invalidates a Mikvah, in order to prevent a person from immersing in a vessel or in a Mikvah filled only with Mayim She'uvim (both of which are invalid according to Torah law; see previous Insight). When a vessel is fashioned from an object already attached to the ground, the Rabanan did not apply their Gezeirah because the reasons for the Gezeirah are not applicable. A person who immerses in a valid Mikvah, into which water was brought via a pipe that was fashioned only after it was attached to the ground, will not mistakenly think that he may immerse himself in a vessel. Since the pipe is firmly attached to the ground, he would not confuse it with immersing in a vessel. The Gemara asks that if the reason why the Gezeirah does not apply is that the pipe is attached to the ground, then a pipe that was fashioned into a pipe before it was attached to the ground also should not invalidate a Mikvah, since it is also presently attached to the ground. The Gemara answers that because the pipe was called a vessel before it was attached, the Gezeirah applies. However, in both cases (a pipe attached before or after it was fashioned), the Rabanan maintain that the vessel is not deemed attached to the ground. (Y. MONTROSE)