Eruvin, Chapter 4, Mishnah 10. Following the two previous mishnayos, in which we learned that a person can designate a particular spot for himself and make it his place of rest for Shabbos, our mishnah teaches a further law on this subject.
The case:
A person leaves his house in order to reach another town where an eruv is placed, and where he has a second home. The distance between the two towns is slightly less than four thousand amos, and therefore he can only reach his home in the other town by placing an eruv midway along the road between the two towns. He set out on this journey, and not only for his own sake but on behalf of other people as well.
On the way, however, his friend met him and turned him back, preventing him from placing the eruv in its spot. He argued that the weather was going to be bad and the like, and persuaded him not to do it. Afterward, though, he changed his mind and decided that he did want to go. What is his status?
The view of Rabbi Yehudah:
He himself is permitted to go - since he set out on the road and began walking toward that spot in order to make an eruv there, and his intention was that his place of rest should be there, he is considered "mafsik baderech" - one who had already stopped along the road. He was already on the way, and his intention was that he should have an eruv at that spot located between the two towns, and therefore he is permitted to go.
The other people are forbidden - all those people on whose behalf he wanted to place the eruv are forbidden, since they were not "mafsik baderech" and did not actually set out on the road. Only one who actually sets out on the road can choose a spot for himself and make an eruv there, as we learned in the previous mishnah.
The view of Rabbi Meir:
Rabbi Meir disagrees and holds that the matter remains in doubt: did he in fact decide to rest at that spot, or was his true intention to remain in his house? Therefore Rabbi Meir says: anyone who could have made an eruv and did not make one, as in the case before us, is like a "chamar gamal" - a donkey driver and a camel driver at once: the donkey pulls him in one direction and the camel in the other, and he ends up stuck between the two of them.
In practice, he may walk only within the area between his house and the spot where the eruv was supposed to have been placed, a spot located slightly less than two thousand amos from his house. Between his house and the presumed eruv he may walk, but beyond these two points, whether in the direction of the second town or on the other side of his house, he may not, because it is unclear where his place of rest is, at which of these two locations. The result is that he has only the two thousand amos between the two spots.
In summary: Our mishnah taught that one who sets out on the road to place an eruv and whose friend turned him back - according to Rabbi Yehudah he himself is permitted to go, since he was already "mafsik baderech" and his intention was to rest at that spot, while the other people on whose behalf he wanted to make the eruv are forbidden, because they did not actually set out on the road. Rabbi Meir holds that the matter remains in doubt, and anyone who could have made an eruv and did not make one is like a "chamar gamal," and he may walk only within the two thousand amos between his house and the location of the presumed eruv.