Zavim, Chapter One, Mishnah Six
Our mishnah deals with a man who has an issue at twilight. The question is does the change of day divide one issue into two issues?
1) If he saw one issue during the day and another at twilight, or one at twilight and the other the next day:
a) If he knew that part of the issue occurred at day-time and part the next day, he is certain with regard to a sacrifice and uncleanness.
b) But if it is in doubt whether part [of the issue] occurred at day-time and part on the next day he is certain with regard to uncleanness, but in one of doubt in with regard a sacrifice.
2) If he had seen issues on two separate days at twilight, he is in doubt both with regard to defilement and with regard to a sacrifice.
3) If [he had seen only] one issue at twilight, he is in doubt [also] with regard to [his] defilement.
Section one: In this case a person had an issue of zov during the day and then another at twilight, a time which is not easy to determine as either night or day. Or he had an issue at twilight and then another the following day. We need to determine whether the issue he had at twilight counts as two issues, such that he is now a full zav. Or does it count as only one, such that he is a zav, but not a full zav?
If he knew that part of the issue that happened at twilight occurred during one day and part the next, then he would certainly be impure and he would certainly be liable for a sacrifice. We learn here that a new day causes one issue to be legally considered two. [We should note that I’m not sure if it’s all that possible to know whether it occurred during one day or the next day. The mishnah wants to teach the principle, the turn of a day separates one sighting from another].
However, if he is not certain whether one occurred one day and the other the next, then there is doubt as to whether he has seen three issues or two. Since in any case he has seen two issues, he is certainly unclean for seven days, as is always the case with a person who saw two issues. However, since it is uncertain whether he has seen two issues or three, he is not liable to bring a sacrifice.
Section two: In this case, he saw an issue for two days straight at twilight. It is possible therefore that an entire day interrupted between the first issue and the second. In such a case he wouldn’t even be impure. His purity status is therefore doubtful. On the other hand, he might have seen three issues, one of them being divided by the turn of the day into two. Therefore, he might be liable for a sacrifice. His status vis a vis a sacrifice is therefore also doubtful.
Section three: One issue seen at twilight is enough to create a doubt with regard to his purity status, for the turn of the day could divide the issue into being considered two issues. However, there is no possibility to consider this to be three sightings. Clearly, he is not liable for a sacrifice.