Taharos, Chapter 10, Mishnah 3. This chapter has been occupied with the workers of the olive press and the grape harvest, people who are commonly amei ha'aretz and yet must be tahor in order to do the job. We have heard a good deal about the requirement that they immerse. What we have not yet heard is what that immersion actually looks like in practice: how closely does the owner have to supervise it? That is the subject of our mishnah.
The mishnah opens: "Habaddadin vehabotzerin, keivan shehichnisan lirshus hame'arah, dayo, divrei Rebbi Meir." The olive pressers and the grape pickers, once he has brought them into the domain of the cave, that is enough, says Rebbi Meir.
What is the "cave"?
The me'arah spoken of here is the cave in which the mikveh is located. Anyone who has gone on a tiyul in Eretz Yisrael, especially in the areas south of Yerushalayim and along the ancient routes leading up to the city, has probably walked past them: caves hollowed into the hillside with ancient mikvaos inside them. It is a striking thing to see, and it is exactly the setting our mishnah is describing.
Three opinions
Rebbi Meir holds that the owner has fulfilled his obligation once he has brought the worker as far as the entrance of the cave. He is not required to walk in behind him and verify with his own eyes that the man went down into the water and immersed properly.
Rebbi Yosei rules more stringently. His words are that the owner "tzarich la'amod aleihen", he is obligated to stand watch over them, and that watch continues "ad sheyitbolu", until the tevilah has actually been performed. Escorting the worker to the doorway and turning around does not discharge the obligation; the employer keeps his eyes on him until he has seen him lower himself into the water. Such a man is not credited with having immersed properly, and quite possibly not with having immersed at all.
Rebbi Shimon rules that everything hinges on the worker's own assessment of himself. "Im tehorin lahen", where these people consider themselves and their possessions to be tahor, then the owner "tzarich la'amod aleihen", he is required to keep watch, "ad sheyitbolu", until they have gone under. The reason is precisely that in their own eyes nothing needs correcting. A man who sees no deficiency in himself feels no urgency about the immersion, and therefore cannot be relied upon to perform it seriously. "Im temei'in lahen", but where they hold themselves to be tamei, and that assessment is accurate, then "ein tzarich la'amod aleihen", no such supervision is called for. Here it is enough to escort him as far as the cave's entrance, exactly as Rebbi Meir maintained, since a man who is well aware of his own tumah will attend to his tevilah on his own.
The halachah
Rebbi Shimon's compromise is not the position we adopt. The halachah follows Rebbi Yosei: the owner must remain there for the entire time, standing over the workers until he has seen them immerse.