The Mishnah discusses the definitions of a maayan, mikveh and all things in between. The mikveh is a collection of still, non-drawn water that is forty seah in volume. The maayan (spring) differs in that it there is no minimum measure (kol shehu) and it can also purify while its water flows (zochalin). One body of water whose classification is in between the maayan and mikveh is a maayan whose own volume of water has been outweighed by drawn water (or rain water). The Mishnah teaches that it is similar to a mikveh in that it can only be a source of purification if the water is collected, yet is nonetheless similar to a maayan in that there is no minimum volume requirement.
The Ran (Nedarim 40b) asks that Mishnah appears to contradict itself. How can one body of water be considered both like a mikveh and like a maayan? If it can purify with any volume it should also be able to while flowing? The Ran therefore explains that the Mishnah must be referring to two different cases; one where it is similar to a mikveh and the other where it is similar to a maayan.
The Ran explains that if the maayan was not flowing before the drawn water was introduced then the mixture is classified in all senses as a mikveh – it requires forty seah and can only purify when the water is collected and still. If however the maayan was initially flowing, then despite the fact that its volume is later outweighed by drawn water, its status as a maayan is unchanged.
The Rambam (Mikvaot 9:6) however does not explain that our Mishnah is referring to two different cases. Accordingly, what is the basis for the distinctions? R' Chayim (Al HaRambam) initially suggest that indeed the law of kol shehu and zochalin can be split. The Mishnah later (5:6) teaches that a wave that detaches from the oceans and lands on someone can cause them to become tahor provided that water contains a volume of forty seah. We find that once the water becomes detached from the ocean it requires a shiur. Nonetheless it appears that it can purify even though it is flowing. Therefore in that single case we find that two laws can be independent of one another (even though the result in this case turns out to be the reverse of our own).
Based on the above case, R’ Chaim explains that we find that when the water is attached to the maayan there is no requirement on the volume. It must however be attached. Consequently, in our case the water, despite containing a majority of drawn water, that water is attached to the maayan and therefore a kol shehu is enough. The law that maayan water can purify while flowing however is not dependant on whether the water is still attached. Rather it is dependent on whether the water is defined as maayan water. In our case since the drawn water outweighs the maayan water, it is treated as a mikveh in that it only purifies when still.
R’ Chaim however rejects this distinction. The reason is that the Rambam (Mikvaot 9:18) explicitly states that one cannot immerse an item in a wave while it is in the air, even if it contains a volume of forty seah, is because “one cannot immerse in zochalin, and therefore even more so in the air.” After some deliberation R’ Chaim concludes that there is no clear proof that one can differentiate between the laws as described above.
Instead R’ Chaim offers a different explanation by reassessing the requirement of forty seah in a mikveh. Initially one might have assumed that the water of a mikveh and maayan are different. For mikveh water to be “potent” enough a volume of forty seah is required. R’ Chaim explains that this is not the case. There is nothing wrong with the mikveh water per se; instead there is a technical requirement of a particular volume. A maayan however does not have that requirement. Consequently, since maayan water of a minimal volume is able to complete its own volume, so too in our case the water of a maayan can complete the required forty seah volume. Accordingly in our case, the mixture is not defined as a mikveh and maayan. The mikveh water (defined so since the drawn water is attached to the maayan) is in the majority and the mixture is therefore defined as a mikveh and can only purify when still. So why can it purify with a minimal volume? Simply because the water of the maayan with which it is mixed, can complete the volume requirement.
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