Eruvin, Chapter 4, Mishnah 5. When a person owns an object, his own Shabbos boundary affects his belongings as well, and those objects are limited to the boundary that their owner has personally. Our Mishnah revolves around the question: what is the law regarding an object that has no owner at all, an ownerless object that belongs to no one? Does it automatically acquire a boundary of two thousand amos, or does it have no boundary at all and is therefore limited to four amos alone?
One Who Fell Asleep Along the Way:
Based on this question, the Mishnah discusses the case of a person who was asleep when Shabbos began: "Mi sheyashan baderech" - one who fell asleep along the way, "v'lo yada shechasheichah" - and did not realize that night had fallen. He assumed he would wake up before Shabbos began, but that is not what happened: he was asleep at the moment Shabbos entered, and now he wakes up and it is already Shabbos. The question is whether he acquired a resting place, that is, whether he established a place for himself while asleep and therefore has two thousand amos, or not.
The Tannaim disagree about this question:
Rabbi Yochanan ben Nuri: He has two thousand amos. Rabbi Yochanan ben Nuri holds that even ownerless objects, which belong to no one, acquire a resting place and have two thousand amos; and a sleeping person is no different from ownerless objects - he too, as it were, belongs to no one but himself, and his status is identical. Just as an ownerless object has two thousand amos, so too this person, who was unconscious and motionless at that moment, acquires a resting place.
The Sages: "Ein lo ela arba amos" - he has only four amos. This is their view regarding ownerless objects as well - they do not acquire a resting place, they have no place of their own, and they are therefore limited to four amos. So too the sleeping person, upon waking, is limited to four amos.
Where Exactly Are the Four Amos Located?
From here the Mishnah moves on to the question: one who has only four amos, where is he positioned in relation to them?
Rabbi Eliezer: "V'hu b'emtza'an" - the person stands in the middle of the four amos, which means that he may walk two amos in each direction. This is unlike the view of the Sages mentioned before him, who said "he has only four amos" and held that he has four amos in every direction, so that in practice we are speaking of eight amos across.
Rabbi Yehudah: In whichever direction he wishes to walk, he may walk four amos, such that he stands at the starting point of the four amos, and he is the one who determines where the four amos will be, according to the direction in which he decides to walk. Nevertheless, Rabbi Yehudah agrees that once he has chosen a particular direction, he cannot retract and turn in the other direction; from that point on he is limited. He has the right to decide in which direction he will go, but once he has decided that this is the direction in which his four amos will be, he can no longer change his mind.
In summary: This Mishnah makes the law of one who fell asleep and was overtaken by Shabbos depend on the status of ownerless objects. According to Rabbi Yochanan ben Nuri, just as ownerless objects acquire a resting place and have two thousand amos, so too the sleeper; and according to the Sages, just as ownerless objects do not acquire a resting place, so too the sleeper has only four amos. The Mishnah then records a dispute about where those four amos are located: according to Rabbi Eliezer the person stands in their middle and has two amos in each direction, and according to Rabbi Yehudah he may choose a direction and walk four amos in it, provided that he does not retract after having chosen.