Eruvin, Chapter 2, Mishnah 4. The Mishnah continues to discuss the laws of passei be'eiros - those upright right-angled boards that are set around a well and render the area surrounding it fit to be treated as a private domain.
The dispute between Rabbi Yehudah and the Sages concerning a public thoroughfare:
"Rabbi Yehudah omer: im haysah derech harabim mafsikasan - yesalkenah litzdadin" - if a public thoroughfare passes right through the passei be'eiros and interrupts between them, it must be shifted off to the sides so that it runs outside the enclosed area. The reason: passei be'eiros are not true, solid partitions, but only something that resembles partitions, and since most of the perimeter is open, a public thoroughfare is able to nullify them and to prevent the enclosure of boards from functioning as real walls and from being considered a private domain.
"VaChachamim omrim: eino tzarich" - in the view of the Sages there is no need to shift the public thoroughfare off to the sides. The fact that the public passes through the area does not prevent it from serving as a private domain, and therefore one may draw water out of the pit, which is a private domain, into the area enclosed by the passei be'eiros, and the water remains within a private domain.
The difference between a pit and a well, and between public and private:
A pit - standing water. This is not flowing water, and therefore it is liable to run dry easily.
A well - a permanent water source, which does not tend to run dry under ordinary circumstances.
Belonging to the public - members of the public remind one another, and if the water runs out they will know not to rely on the boards.
Belonging to an individual - a private person may fail to notice that the water has run out, and go on assuming that he is permitted to carry.
The halachah is that passei be'eiros are effective only when there is an active, available well. Once there is no water in it, the boards no longer function to create a private domain, and one who draws from the pit and sets the water down alongside it desecrates Shabbos. Hence the distinction between public and private: with an individual there is a concern that he will not remember, whereas among the public one reminds the other.
The view of Rabbi Akiva:
The first Tanna rules: "Echad bor harabim uv'er harabim uv'er hayachid - osin lahen passin" - in all of these the laws of passei be'eiros apply:
A public pit - even though the water is liable to run out, since it belongs to the public they will remind one another when the water runs dry.
A public well - the water is not liable to run out.
An individual's well - even though it belongs to an individual, the water is not liable to run out, since this is a well fed by a flowing spring.
"Aval l'vor hayachid - osin lo mechitzah gevohah asarah tefachim, divrei Rabbi Akiva" - in the case of an individual's pit, which is standing water that is liable to run out, and where he is liable not to notice that the water has run dry and that the boards are no longer effective, there a real, solid partition ten tefachim high is required. This is the opinion of Rabbi Akiva: passei be'eiros are permitted for a public pit, a public well and an individual's well, but not for an individual's pit, because in an individual's pit two drawbacks combine: the water is liable to run out, and he is an individual who is liable not to notice this.
The view of Rabbi Yehudah ben Bava:
"Rabbi Yehudah ben Bava omer: ein osin passin ela liv'er harabim" - Rabbi Yehudah ben Bava is more stringent: passei be'eiros are effective only for a well whose water does not run dry, and only when it belongs to the public. In the other three cases a real partition ten tefachim high must be made.
In summary: in this Mishnah Rabbi Yehudah and the Sages disagree about a public thoroughfare that passes between the passei be'eiros - whether it must be shifted off to the sides because the boards are not full partitions, or whether there is no need for that. We then clarified the two determining factors: a pit (standing water liable to run dry) as opposed to a well (a permanent source), and the public (who remind one another) as opposed to an individual (who is liable to forget). According to Rabbi Akiva, passei be'eiros are effective for a public pit, a public well and an individual's well, and only for an individual's pit, where the two drawbacks combine, is a partition ten tefachim high required; and according to Rabbi Yehudah ben Bava, boards may be used only for a public well, while in all the other cases a real partition is required.