If a person is traveling on Friday afternoon and realizes that it is almost Shabbat, the Mishna teaches that he can declare that he is establishing his Shabbat “at the trunk of the tree” that is 2,000 amot away, and he is then able to walk to that tree and continue to his house that is 2,000 amot beyond the tree. If, however, he said that he is establishing Shabbat “beneath the tree,” the Mishna teaches that he accomplishes nothing, since his statement was not clear enough.
The Gemara discusses what the Mishna means when it says that nothing is accomplished in the case where the man says that he is establishing his Shabbat “beneath the tree.”
Rav said: He has not said anything at all, and has failed to establish residence anywhere, and he may not even go to the place beneath that tree. His failure to specify a particular location prevents him from establishing residence beneath the tree. The fact that he sought to establish residence someplace other than his present location prevents him from establishing residence at his present location. Accordingly he may walk no more than four cubits from the place that he is standing.
And Shmuel said: He has not said anything with regard to going to his home, if it is two thousand cubits past the tree; however, with regard to the area beneath the tree, if its bough is entirely within two thousand cubits of his present location he may indeed go there.
At that point, according to Shmuel, he becomes the proverbial “donkey-camel driver” (see daf 35) who is limited by his situation to just 2,000 amot between the two points.
Rashi brings two approaches to Shmuel’s ruling:
When a person declares that he is establishing his Shabbat “under the tree” it is unclear which part of the tree he is choosing. Since his house is 2,000 amot beyond the trunk of the tree, perhaps he is establishing his Shabbat at the edge of the tree that is too far away from his house.
When he declares his Shabbat in a place that is not clearly specified, we are not certain what he means, and therefore it is not clear whether his basic tehum is where he is at the moment of his declaration or if he succeeded in moving it to the tree. He can, therefore, move back and forth between the original spot and the tree, but no further.
Maimonides appears to understand Shmuel’s ruling differently. According to him, the man who makes this declaration is not at all successful in establishing a new place for Shabbat, so he remains with a 2,000 cubit radius from his original point. That allows him to walk to the tree, but no further than that.