Mikvaos, Chapter 6, Mishnah 7. Our mishnah takes up again the topic of eiruv mikvaos, the merging of two pools of water that sit beside one another. Each pool on its own falls short of arba'im se'ah; yet when a proper connection links them, halachah views them as a single hakva'ah, one collection of water, so that the two of them together supply the full measure. What the mishnah comes to define is the dimension of the passage that creates this union.
The measure of the opening
"Eiruv mikvaos kishfoferes hanod, k'ovyah uk'chalalah"
The yardstick is the shefoferes hanod, the spout or plug of a leather flask, the piece that sits snugly in the neck of the vessel. Two dimensions are counted together: k'ovyah, the thickness of its walls, and uk'chalalah, the hollow within it. The mishnah then translates this into something one can picture: a gap wide enough for two fingers, and not two fingers jammed in tightly, but two fingers that are chozros limkoman, able to enter and rotate comfortably, turning this way and that within the space.
A doubt in the measurement
"Safek kishfoferes hanod safek she'eino kishfoferes hanod, pesulah, mipnei shehu min haTorah"
Picture someone who has immersed in one of a pair of neighboring mikvaos. Neither pool contained forty se'ah independently; only their union could have produced the measure. Afterward the size of the passage comes into question: perhaps it was a proper shefoferes hanod, perhaps it was smaller. The mishnah disqualifies the immersion, and it supplies the reason: the matter is min haTorah. Where a Torah requirement hangs in the balance, and the principle is that a safek d'oraisa is treated strictly, we must reckon with the possibility that the passage fell short and that the two pools never became one.
The same principle in the measures of tumah
The mishnah proceeds to show that this very reasoning applies to the shiurim, the minimum quantities, that make something a source of tumah:
"V'chen b'kazayis min hameis, uk'zayis min hanevelah, uk'adashah min hasheretz"
Suppose one cannot determine whether the amount present amounted to a kezayis of a corpse, or a kezayis of nevelah, or the volume of a lentil taken from a sheretz that transmits tumah. Here the strict conclusion points the other way: we treat the quantity as though it had been complete, and tumah as though it had been transmitted. Neither ruling is arbitrary. In both cases we are facing a safek d'oraisa, and such a doubt is always resolved in the demanding direction, whether the outcome is a mikveh ruled unfit or a person ruled tamei.
Something lodged in the opening
The mishnah now addresses a passage that does measure a full shefoferes hanod, but in which some item has become stuck, so that the channel is narrowed.
"Kol sheya'amid kishfoferes hanod, m'ma'ato"
Whatever occupies space in that channel subtracts from its measure. Once the item has reduced the gap so that a full shefoferes hanod is no longer left, the link between the pools has been severed, and they cease to be treated as one body of water.
The view of Rabban Shimon ben Gamliel
"Rabban Shimon ben Gamliel omer: kol shehu mibrias hamayim, tahor"
Rabban Shimon ben Gamliel introduces an exception. Whatever originates in the creation of the water itself, meaning a fish, which the Kadosh Baruch Hu called forth from the waters, is not considered to be blocking anything. Should such a creature come to rest in the channel that joins the pools, it is no chatzitzah. Its halachic character follows that of the water, so the pools stay me'uravin, united, and the necessary width of the passage is deemed undiminished.
Summary
A pair of mikvaos becomes one when the channel joining them measures a shefoferes hanod, that is, the width of two fingers with space enough to turn them. Uncertainty about that width is decided strictly, as is every uncertainty regarding the Torah's measures, and whatever fills the channel diminishes it, apart from a living creature that came out of the water in the first place, in the opinion of Rabban Shimon ben Gamliel.