Zavim, Chapter 2, Mishnah 3. Our chapter is occupied with the question of when a discharge counts as a genuine re'iyah of zivah and when it does not. An earlier mishnah in this perek already established the principle that a flow which comes about through an oines, an outside cause, is not counted toward making a person a zav. This mishnah applies that principle in a case where the outside cause is the man's own seminal discharge, and then it gathers together several other halachos that share one common feature: a span of me'eis le'eis, a full twenty-four hours measured from moment to moment.
A re'iyah followed by keri
"Haro'eh keri eino metamei b'zivah me'eis le'eis": one who has experienced a seminal emission does not become tamei with the tumah of zivah for twenty-four hours. The picture is this: a man had one re'iyah of zivah, and afterward he had a seminal discharge. If a second flow now appears within twenty-four hours of that keri, it is not counted as a second re'iyah for the purposes of tumas zav.
Why not? Because we attribute the later flow to the seminal emission that preceded it. It is treated as a consequence of that discharge rather than as a symptom of zivah, and a flow that comes from such an outside cause is an oines, and an oines does not render a person a zav.
Rabbi Yosi disagrees about the length of the window. In his view we do not measure a full twenty-four hours but "yomo," the remainder of that same day on which the keri occurred. Until that day is over, a second flow is written off as the by-product of the emission and carries no consequence. The moment the day has ended, however, we no longer make that attribution, and a further flow would count toward tumas zav in the ordinary way.
A convert who had a keri before his conversion
"Goy shera'ah keri v'nisgayer, miyad hu metamei b'zivah": a gentile who experienced keri and afterward entered the Jewish people is subject to tumas zivah at once. Picture him now as a Jew, having a flow of zivah while still inside the twenty-four hours that follow that earlier emission. One might have assumed that the rule just learned would cover him too, so that the flow would be charged to the keri and dismissed as an oines. The mishnah rules that we do not say so.
The reason lies in the nature of conversion. A ger who converts is like a newborn child, ger shenisgayer k'katan shenolad dami. As a new person he has no earlier experience to which the present flow can be traced. Whatever happened before the conversion belongs, as it were, to a different life and a different person, and the halachah severs the connection entirely. Since there is nothing to point to as the cause, the flow stands on its own and renders him tamei.
Other halachos measured me'eis le'eis
"Haro'eh dam v'hamakasheh me'eis le'eis." The mishnah now lists further areas of halachah in which this same twenty-four hour measure governs.
The first is a woman who sees blood. Her sighting reaches backward: taharos that she handled during the twenty-four hours before she saw the blood are rendered tamei retroactively, so that any pure food she touched in that span becomes impure. This is the halachah taught in Maseches Niddah.
The second is the makashah, a woman in hard travail. If she discharges blood during her days of zivah while she is in that difficult state, she is not a zavah, because the blood is attributed to the ordeal of her labor rather than to zivah. Here too the measure is twenty-four hours: if she was free of that hard travail for a full twenty-four hours and then gave birth, the attribution no longer holds and she is counted as a yoledes b'zov, one who gave birth while in a state of zivah.
Striking a servant
The mishnah's next item is "hamakeh es avdo," a master who strikes his servant. Parshas Mishpatim teaches that when a man beats his eved or his shifchah with a rod and the victim expires under his hand, the master is subject to punishment; but should the servant remain alive for "yom o yomayim," a day or two, no such penalty is imposed.
Chazal understand the Torah's expression here as a single unbroken span of twenty-four hours, counted from the moment the blow landed. Should death come thirty-six hours afterward, the master is spared that punishment; should it come while those twenty-four hours are still running, he is held liable.
A dog that ate flesh of a corpse
"Kelev she'achal b'sar hameis, shloshah yamim me'eis le'eis, harei hu k'viryaso." A dog that consumed the flesh of a human corpse retains that flesh in its original halachic state for three full twenty-four hour periods, and during that time the flesh inside the dog is regarded as though it were still intact.
The practical result: if the dog dies within those three days, the flesh of the corpse inside it is still capable of conveying tumah, and during that span the dog can transmit tumas ohel, the tumah that spreads under a roof from a corpse. Once seventy-two hours have passed, however, and only afterward does the animal die, we assume that the flesh has been fully broken down and digested. The remains of the human body no longer have their former constitution, and nothing there conveys tumah.
So the mishnah closes with a set of halachos drawn from very different corners of Torah, all bound together by one measure: the eis le'eis, the full twenty-four hours from moment to moment.