Nazir, Chapter 9, Mishnah 4. This Mishnah continues the subject discussed in the previous mishnayos - 'raglayim ladavar', that there are grounds for assuming something. The principle is that once a given matter has been established and a chazakah, a halachic presumption, has been created regarding it, we continue to follow that presumption going forward as well.
A Doubtful Case of Nega'im:
The Mishnah states: "Kol sefek nega'im batechilah tahor, ad shelo nizkak letumah; mishenizkak letumah - sefeiko tamei" - every doubtful case of nega'im is initially tahor, as long as he has not yet become subject to tumah; once he has become subject to tumah, his doubtful case is tamei. In other words: a person about whom there is a doubt whether he has contracted tzara'as, as long as he has not experienced tumah up to that point and he has a presumption of taharah - he is tahor. But once he has become subject to tumah, meaning that he has already been established as tamei, and there is a doubt whether he is still tamei - his doubtful case is tamei, and we assume that he is tamei.
The Mishnah in Maseches Nega'im (Chapter 5) explains which case is being discussed:
The case in which the doubt is tahor: Two people came to the kohen with their nega'im. One had a baheres - a type of tzara'as - the size of a split bean, which is the minimum measure, and the second had a baheres the size of a sela, which is larger. At the end of the week both spots were found to be the size of a sela, and we do not know which of them grew in size. One of them remained at its original measure, and therefore it bears no sign of tumah - since the spreading of the nega (pisyon) is a sign of tumah - and one of the two did not have this sign. Whether we are dealing with one person or with two people, we say that they are tahor, since up to that point they had not been established as tamei.
The case in which the doubt is tamei: Two people came to the kohen, one with a baheres the size of a split bean and one the size of a sela. At the end of the week each of them was found to have a sela and a bit more, so that both had spread, and therefore both were certainly tamei. If afterward both spots receded to the size of a sela, this means that one of them returned to its original size and its spreading disappeared, while the second, which was originally only the size of a split bean, remained larger than it had been. Since both had already been established as tamei, we must be stringent with both and say that both are tamei - even though one of them has certainly become tahor and its spreading has disappeared, we simply do not know which one. This is the example to which our Mishnah refers.
Seven Ways in Which a Zav Is Examined:
A zav is one who has a discharge of zivah. The first sighting renders him tamei, exactly like a ba'al keri, and even if it came about through an accidental cause - it counts as a first sighting of a zav. But the laws of a zav regarding the need to count seven clean days do not apply to him, and that obligation does not take effect until the second sighting. The second sighting renders him tamei only if it did not come about through an external cause, which is called an accident (ones). But once it has been established that the second sighting came from zivah and not from an accident, then with the third sighting it makes no difference whether it came about accidentally or not, since he has already been established as a zav after the second sighting.
The Mishnah states: "Beshivah derachim bodkin es hazav, ad shelo nizkak lezivah" - there are seven ways in which a zav is examined, as long as he has not yet become subject to zivah - seven ways of examining whether the sighting of the discharge came about because he is a zav or because of an external cause. This examination is made only before he has been definitively established as a zav, that is, before he has had two sightings. And these are the seven:
"Bema'achal" - through food: that he ate too much, and it is possible that the food caused it.
"Bemishteh" - through drink: that he drank too much.
"Bemasa" - through carrying: that he carried a heavy load.
"Uvekefitzah" - and through jumping: by means of jumping.
"Uvecholi" - and through illness: while he was ill.
"Uvemar'eh" - and through sight: that he saw a woman, and this sight aroused him.
"Uveharhur" - and through thought: that he had thoughts about a woman, and the thought aroused him.
All seven of these ways apply before he has been established as one who has had two sightings, and they relate to the second sighting.
"Mishenizkak lezivah - ein bodkin oso" - once he has become subject to zivah, he is no longer examined: once he has been established as one who has had two sightings of zivah, the third sighting is not examined to determine whether it came about accidentally or not. And therefore "onso usefeiko veshichvas zaro" - his accidental sighting, his doubtful sighting and his seminal emission:
"Onso" - even if he had a sighting due to an accident, because of an external cause.
"Usefeiko" - even if we are in doubt whether the third sighting was a sighting of zivah, or perhaps it was nothing more than a seminal emission.
"Veshichvas zaro" - the seminal emission of one who has become a zav is considered tamei.
All of these are tamei, "sheraglayim ladavar" - for there are grounds for the assumption: once he has had two definite sightings of zivah, we assume that he is a zav with the third as well, regardless of how it came about.
One Who Strikes His Fellow and He Was Assessed as Fatally Injured:
The third case in the Mishnah: "Hamakeh es chaveiro va'amaduhu lemisah" - one who struck his fellow, and the doctors assessed that he would die from the blow. "Vehekeil mimah shehayah" - the injury caused by the blow improved, and the doctors thought that he would live. "Le'achar michan hichbid umeis" - afterward his condition worsened again, and in the end he died. "Chayav" - the one who struck him is liable for killing him. "Rabbi Nechemyah omer: patur, sheraglayim ladavar" - Rabbi Nechemyah says that he is exempt, because there is a chazakah here: from the fact that his condition began to improve it is evident that his death did not result from the original blow but from some other problem, for the doctors had thought that he was going to live.
And some explain the concluding words "sheraglayim ladavar" not as support for the position of Rabbi Nechemyah, who exempts, but as an explanation of the position of the first Tanna, who holds him liable: since he was initially assessed as fatally injured, that determination and that presumption remain with him. And even though at a certain stage they thought otherwise, since at the outset they thought he was going to die and in the end he indeed died - he is liable, for there are grounds for the assumption: there is a presumption that his death resulted from the original blow.
In summary: In this Mishnah we learned three cases that apply the principle of chazakah and 'raglayim ladavar': a doubtful case of nega'im - that before he has become subject to tumah his doubtful case is tahor, and once he has become subject to tumah his doubtful case is tamei; the seven ways in which a zav is examined - which apply only before he has become subject to zivah, and once he has become subject to it his accidental sighting, his doubtful sighting and his seminal emission are all tamei; and one who strikes his fellow and he was assessed as fatally injured, where they disagree as to whether the presumption teaches that he is exempt (Rabbi Nechemyah) or liable (the first Tanna).