Mikvaos, Chapter 3, Mishnah 2. Our chapter deals with mayim she'uvim, drawn water, water that was gathered in a vessel. The chachomim established that if three lugin of such water fall into a mikveh before it has reached its full forty se'ah of rainwater, the mikveh is disqualified, and it remains so even after it later fills up. According to the chachomim, one path back to validity was set out: le'olam hu bifsolo ad sheyeitzei mimenu m'loyo va'od, it stays invalid until its full measure has flowed out of it and something more. Our mishnah opens with the practical question, keitzad, how is that carried out?
The scenario presented is habor shebechatzer, a pit dug in a courtyard, into which naflu lo shelosha lugin, three lugin of drawn water made their way while the rainwater collected there was still under forty se'ah. The ruling given is le'olam hu bifsolo, the disqualification clings to it permanently, and the pit recovers only ad sheyeitzei mimenu m'loyo (its entire contents, all forty se'ah, must first be pushed out by fresh rainwater streaming in) va'od, with a further amount leaving beyond that. Why insist on the additional bit? Draining out precisely forty se'ah would deal with the disqualified body of water itself, yet the three lugin of drawn water would still trouble us, since they may well be lying somewhere within the pit. Once the outflow exceeds the full measure, we are willing to assume that part of those three lugin was carried out too, so that three intact lugin are no longer present and the water originally ruled unfit has departed. Nothing remains but new rainwater, and the pit functions as a valid mikveh.
The mishnah then offers a second route: o ad sheya'amid bechatzer arba'im se'ah, or until he sets up in the courtyard a body of forty se'ah. That is, he digs another hollow elsewhere in that same courtyard and lets it fill with forty se'ah of proper rainwater. Then veyit'haru ha'elyonim vehatachtonim, and the upper waters and the lower waters become pure. Picture the courtyard on a slope: the higher pool is the valid one, while the lower one is the pit into which the three lugin had fallen. A stream runs from the upper to the lower, joining them, and through that connection the valid mikveh renders the invalid one fit. This is the mechanism known as hashakah, and the Masechta will treat it at greater length further on. Its principle is that a mikveh which is not fit can be made fit through contact with a mikveh which is.
A dissenting view follows: Rebbi Elazar ben Azaryah poseil, this Tanna holds that the pit stays unfit. He is not challenging hashakah in principle, for when two mikva'os lie flush against one another he agrees that the contact does its work. His difficulty is with the layout described here, where a stretch of courtyard separates the two pools. To regard them as one body of water one would have to lean on goud asik or goud achis, conceptually raising the lower pool or bringing the higher one down so that the pair counts as a single entity. That construct he refuses to apply once a gap lies between them, and consequently the channel running from one to the other achieves nothing in his opinion.
His statement ends with the phrase ella im kein pokak, which the mefarshim understand as though it read posak, sending us back to the remedy with which the mishnah began, m'loyo va'od. The meaning is that in the view of Rebbi Elazar ben Azaryah a single repair is available for such a pit: new water must pour in and force the old water out, until it is established that a full forty se'ah has left together with a portion of the three lugin. From that point on we may treat what stands in the pit as authentic rainwater, and it serves as a kosher mikveh.
So Mishnah 2 spells out the two ways a disqualified pit in a courtyard can be brought back: displacement of its entire contents and a bit more, or joining it to a proper forty se'ah through hashakah, with Rebbi Elazar ben Azaryah restricting the case to the first path alone when the two pools stand at a distance from one another.