Before us is chapter 4 of Maseches Eruvin, mishnah 1. This mishnah deals with the law of a person who left the boundary within which he began Shabbos, when this happened against his will and under compulsion.
"Mi she'hotzi'uhu goyim o ruach ra'ah - ein lo ela arba amos" - one whom gentiles or an evil spirit took out has only four cubits:
A person who was taken out of his boundary on Shabbos beyond two thousand cubits, whether by gentiles or because he was seized by an evil spirit, and his removal happened against his will, has only four cubits when he comes back to himself, or from the moment he arrives at the place to which he was brought, and he may not move beyond them.
"Hechezoruhu - ke'ilu lo yatza" - if they brought him back, it is as though he never left:
If they returned him to his place, it is as though he never left at all, and therefore he has two thousand cubits from his place of residence or from the city in which he finds himself, that is, two thousand cubits from the edges of that city. However, the Gemara stresses that this is true only when both his removal and his return took place under compulsion:
Taken out under compulsion and returned under compulsion: "as though he never left," and he has two thousand cubits as at the outset.
Taken out under compulsion and returned of his own will: he has only four cubits, and he does not regain his original status. And if he was returned into a city, he may still walk throughout the entire city, such as within the city walls or in an area surrounded by partitions, but he does not have the two thousand cubits outside it.
"Holichuhu le'ir acheres, o nesanuhu b'dir o sahar" - if they led him to another city, or placed him in a pen or an enclosure:
Here the case is one whom gentiles took out of his city and beyond his boundary and brought to another city that is surrounded by its own walls, or whom they brought into a pen or an enclosure, a place designated for sheep, which also have partitions. In this law the Tannaim disagreed:
Rabban Gamliel and Rabbi Elazar ben Azaryah say "mehalech es kulah" - he may walk through all of it: he is permitted to walk throughout the entire area, since any area surrounded by partitions is regarded for him as four cubits.
Rabbi Yehoshua and Rabbi Akiva say "ein lo ela arba amos" - he has only four cubits: since he was not within these partitions when Shabbos began, the partitions are of no help to him, and he is limited to four cubits. According to their view, since he was not within these partitions before Shabbos, his status is that of one standing in an area that has no partitions, just as in the first clause of the mishnah, where he was taken beyond his boundary and has only four cubits.
Some understand that the stringency of Rabbi Yehoshua and Rabbi Akiva, that he has only four cubits, is based on a decree, a decree on account of an area that has no partitions.
The incident that the mishnah brings:
These four sages came from a place called Pardesin. Their ship had already entered the harbor on Friday afternoon, but it was drawn back out into the sea, and they now found themselves outside the boundary. Rabban Gamliel and Rabbi Elazar ben Azaryah walked the entire length of the ship, for although they had been taken out of the boundary under compulsion, they were within partitions, and according to their view one who is within partitions may walk through the entire area. Rabbi Yehoshua and Rabbi Akiva, who disagreed with them, did not move from their four cubits within the ship, because they wished to be stringent with themselves.
A precise reading of the conclusion of the incident:
The emphasis that they wished to be stringent with themselves teaches that what they did was not the strict law, even according to their own view. And why does their conduct not match their opinion? For in their opinion, one who was brought under compulsion on Shabbos into an area that has partitions has only four cubits. The answer is that this ruling was stated because in that scenario he was not within the partitions before Shabbos. In the case of the ship, by contrast, the sages were within those very same partitions already before Shabbos, and therefore by law the partitions are indeed effective and valid for them, and they merely wished to be stringent with themselves.
In summary: in this mishnah we learned that one who was taken out of his boundary under compulsion has only four cubits; that a return under compulsion places him in the position of one who never left, whereas a return of his own will leaves him with four cubits (or within the enclosed city alone); the dispute of the Tannaim regarding one who was brought to another city or to a pen and an enclosure; and the incident of the ship, from which we learned that the conduct of Rabbi Yehoshua and Rabbi Akiva was a stringency they accepted upon themselves and not the strict law.