Eruvin, Chapter 2, Mishnah 3. In the previous Mishnah we reached the conclusion that the area enclosed by the upright boards around wells may be expanded as much as is needed, as long as we have enough boards and planks available so that no gap of more than thirteen and a third amos, or ten amos, is left between the planks. In the Mishnah before us we will present an opinion that holds there is a limit to the matter.
The opinion of Rabbi Yehudah:
"Rabbi Yehudah omer: ad beis sa'atayim" - one may spread the boards around the well only up to the point where the area enclosed between them amounts to a beis sa'atayim, which is five thousand square amos.
The response of the Sages:
"Amru lo: lo amru beis sa'atayim ela l'ginah ul'karpef" - the Sages disagree and hold that there is no limit here at all. The limit of a beis sa'atayim was stated in one case only: when enclosing a garden or a karpef.
Garden - a planted area.
Karpef - an area set aside outside the city for storing wood and the like.
Both of these are considered areas that were not enclosed for the purpose of dwelling - they are not enclosed for living in, but for storage alone. Therefore a limit applies to them: in the absence of real partitions, the area may not exceed a beis sa'atayim, that is, five thousand square amos.
"Aval im hayah dir o sahar, muktzeh o chatzer":
Dir - a place for animals.
Sahar - also an area used for animals.
Muktzeh - a sort of back yard behind the houses.
Chatzer - the courtyard of a house.
Regarding these areas it is said "afilu beis chameishes korin" - a kor is thirty se'ah, and five korin are a hundred and fifty se'ah. An enormous area, instead of only two se'ah. And not only that: "afilu beis aseres korin" - three hundred se'ah, an enormous area that may be enclosed. And all this because it was enclosed for the purpose of dwelling: these areas are considered a place of living, for even in an animal pen there is a spot where the shepherd stays. Since these are places that a person uses and resides in, they may be as large as one likes, even to an extraordinary measure.
Applying this to the boards around wells:
In the view of the Sages, the same law is true of the boards around wells as well: they too are considered an ordinary dwelling area, since this is an ordinary human function and activity - watering one's animal, drawing water for his own needs and the needs of his household. Therefore this is a functional area that is considered enclosed for the purpose of dwelling, and consequently it may be expanded as much as is needed and is not limited to a beis sa'atayim as Rabbi Yehudah holds.
In summary: Rabbi Yehudah and the Sages disagree about limiting the area inside the boards around wells. According to Rabbi Yehudah the area is limited to a beis sa'atayim; and according to the Sages this limit was stated only regarding a garden and a karpef, which are not enclosed for dwelling, whereas a dir, a sahar, a muktzeh and a chatzer - and likewise the boards around wells - are enclosed for dwelling, and therefore one may set them as far apart as is needed, even a beis chameishes korin and even a beis aseres korin. And as we learned in the previous Mishnah, all of this is on condition that additional boards are added so that the gap between the planks is maintained - ten amos or thirteen and a third amos - each according to the views over which Rabbi Meir and Rabbi Yehudah disagreed.