Mikvaos, Chapter 3, Mishnah 4. This chapter deals with the rabbinic takanah that three lugin of mayim she'uvim, drawn water, falling into a mikveh that does not yet contain forty se'ah of mei geshamim, disqualify it. Our mishnah records the Chachomim's own qualifications of that decree, teaching us how far it extends and where it stops.
What matters, say the Chachomim, is how many utensils supplied that water. If the entire three lugin arrived from a single utensil, or was divided between two, or among three, the amounts are added to one another: "mikli echad," from one vessel, "o mishnayim umishlosha keilim mitstarfin," or from two and three vessels, they combine. The water is then viewed as one quantity of three lugin entering the mikveh, and the mikveh is unfit. "Ume'arba'ah ein mitstarfin": water contributed by four separate utensils is not added together at all, and the mikveh remains kosher. Disqualification thus results only where the three lugin issued from one utensil, or from two, or from three. Here too the explanation lies in the character of this halachah: the psul is an enactment of the Chachomim rather than Torah law, so it operates exactly along the lines they drew, and they never extended it to water that came by way of a fourth utensil.
At this point the mishnah steps away to two other halachos in which drawn water plays a role, and at the end it will return to our subject and teach further leniencies concerning the falling of three lugin into a mikveh.
Ezra Hasofer instituted that one who is tamei with tumas keri may not learn Torah until he immerses in a mikveh. Although divrei Torah are not susceptible to tumah, an injunction was nevertheless placed upon him to refrain from learning until he had toiveled. This takanah was later annulled, since it proved to be a gezeirah she'ein efshar la'amod bah, a decree the community could not sustain, though there are individuals today who are still careful with tevilas Ezra. Already in Ezra's time it was established alongside the takanah that a choleh, one who for reasons of his health was unable to immerse in a valid mikveh, would be permitted to learn divrei Torah if nine kabin of drawn water were poured over him.
A second enactment of the Chachomim concerns someone who at present carries no tumah at all. Should three lugin of drawn water land upon his head, or upon the larger part of his body, he acquires tumah, and his status is that of a sheini l'tumah. The practical outcome is that terumah he handles is ruined, for terumah touched by a sheini becomes a shlishi and is thereby unfit.
These two halachos are now folded into the mishnah's ruling about utensils. "Ba'al keri hacholeh shenoflu alav tish'ah kabin mayim": an ailing ba'al keri who had nine kabin poured over him. "V'tahor," and one who bears no tumah, upon whose head, or upon the greater part of whom, there fell "shlosha lugin mayim she'uvim," three lugin of drawn water, leaving him unfit to handle terumah. In both cases the same limit applies: "mikli echad," "o mishnayim umishlosha mitstarfin, me'arba'ah ein mitstarfin." Three is the maximum number of utensils whose separate pourings we still treat as a single act. The sick man therefore achieves his taharah provided the nine kabin were supplied by no more than three utensils, and the man who was tahor contracts tumah provided the three lugin were supplied by one, two or three. Where the water was drawn from a fourth utensil as well, nothing combines, and his status is unchanged.
The mishnah now circles back to its opening ruling and attaches two indispensable conditions to it. "Bameh devarim amurim": in which circumstance did we rule that a second and a third utensil join with the first? "Bizman shehischil hasheini," where the flow from the next utensil had already started "ad shelo pasak harishon," while the first was still running. Overlapping streams count as one pouring, and only under those circumstances are the amounts added to one another.
A second qualification follows, introduced once more by the words "bameh devarim amurim": in which situation must the later pouring overlap the earlier one, and in which situation is a fourth utensil excluded from the reckoning altogether? "Bizman shelo hiskavein l'harbos," where the one introducing the water had no aim of enlarging what the mikveh held and was merely putting water in. "Aval im hiskavein l'harbos," but where his very purpose was to raise the water level, and that is why he added it, then "afilu kortov b'chol shanah mitstarfin l'shlosha lugin": even the tiniest amount at a time, dripped in over the course of a full year into a mikveh that lacks forty se'ah, is all reckoned together. Once those bits of drawn water total three lugin, the mikveh is invalidated, despite the enormous stretch of time across which they entered.
As to what "miskavein l'harbos" actually means, there are various shitos among the Rishonim. The most widely accepted explanation, and the one for which we are choshesh, is that of the Rashba, who holds that whenever the water is poured in b'kavanah, deliberately, as opposed to falling in on its own, that itself constitutes miskavein l'harbos.