Zavim, Chapter 1, Mishnah 2. Our masechta treats the laws of the zav, a man who suffers a flux emerging from an eiver meis, an organ without vitality. That is to be distinguished from an ordinary emission of semen, which leaves a living organ and renders a man a ba'al keri, a status governed by an entirely different set of rules. Once a man has had two sightings of zov, he is obligated to assemble seven days wholly free of any such flow, the shiv'ah nekiyim, before he can shed his impurity. Our mishnah asks what becomes of that tally when something intervenes in the middle of it.
Beis Shammai and Beis Hillel
The mishnah opens: "Haro'eh keri", one who has a seminal emission, "b'yom hashlishi", on day three, "lisfeeras zovo", of the counting of his zov. Picture a man who has already had his two sightings and has begun his tally of clean days. Then, on day three, what issues from him is not zov from a lifeless organ but plain semen from a living one, a keri. Beis Shammai rule: "soser shnei yamim shelefanav".
In the view of Beis Shammai, this keri wipes out the pair of days he had already banked. His tally collapses, and he must open a fresh set of seven clean days from the start.
Beis Hillel disagrees: lo soser ela yomo. The keri undoes only the day on which it occurred. The two earlier days stand, and he continues on, counting another five days to complete the seven.
Rebbi Yishmael
"Rebbi Yishmael omer, haro'eh vasheini, soser es shelefanav." Rebbi Yishmael takes up a case in which the emission arrives sooner, on day two of the shiv'ah nekiyim. Trace the order of events: the sightings of zov from an eiver meis came first, the man then embarked on his clean days, and now, one day into the count, semen issues from him. In such a case, teaches Rebbi Yishmael, even Beis Hillel grants that the single day already counted is cancelled, and the man begins his seven clean days over from scratch.
Rebbi Akiva
"Rebbi Akiva omer": Rebbi Akiva will have none of that distinction. "Echad haro'eh vasheini", whether the emission came on day two of the shiv'ah nekiyim, "v'echad haro'eh vashlishi", or on day three, one and the same machlokes is at issue. "SheBeis Shammai omrim", for Beis Shammai maintain in either case, "sosar shnei yamim shelefanav": the days already accumulated fall away and the seven must be counted afresh. "Beis Hillel omrim", while Beis Hillel hold in either case, "lo sosar ela yomo": nothing at all is forfeited beyond the day of the emission itself. Whatever days he has already put toward his goal of seven days untouched by any flow from an eiver meis stay in place, and he resumes exactly where he stopped.
Where all agree
The mishnah ends with a case of agreement: "U'modim baro'eh bar'vii", all concede regarding one who has an emission on the fourth day, "shelo sosar ela yomo", that nothing beyond that one day is lost. Here Beis Shammai come around to the position of Beis Hillel. Our man is a full-fledged zav in the midst of his seven clean days; three days have gone by without incident, and on day four semen issues from him. Both houses hold that he forfeits that day alone and then counts four more full days to round out the seven.
Observe, though, how narrow this leniency is. It holds only baro'eh keri, only where what appeared during those days was semen. "Ra'ah zov", should he see actual zov, "afilu b'yom hashvi'i", even as late as the seventh day, then "soser es shelefanav", all that lies behind him is undone and he begins again. The logic needs little elaboration: what is demanded is a stretch of seven days wholly untainted by zov, with not a drop issuing from an eiver meis. Only when seven such days have been gathered together is purification from the status of a zav gadol possible, so a flux on the final day ruins the count no less than one on the opening day.
So this mishnah teaches us to weigh two different kinds of emission against one count. A keri is an interruption whose reach is disputed, wide according to Beis Shammai, narrow according to Beis Hillel. A discharge of zov, however, is not an interruption at all but a return to the very impurity the count was meant to leave behind, and against it there is no dispute.