1) A MAN WITHOUT A "MAKOM SHEVISAH"
QUESTION: The Mishnah says that when a person attempts to make his Makom Shevisah underneath a tree and he does not specify exactly where he wants his Makom Shevisah to be, his attempt is ineffective. Rav explains that he has no Makom Shevisah anywhere; he has Shevisah neither under the tree nor at the place where he is standing.
The Mishnah earlier (35a), however, seems to contradict Rav's assertion. The Mishnah says that when a person attempts to make an Eruv Techumin at the end of 2000 Amos from his present location and the food rolls beyond the 2000 Amos before Shabbos enters, the Eruv is not valid, and the person retains the Techum of his city. According to Rav, though, he should not have any Techum!
ANSWER: TOSFOS (DH v'Afilu) and RASHI (60a, DH v'Asur) answer that when a person is standing inside of a city, he always has in mind that if his Eruv is ineffective for some reason, he will retain the Techum of the city (as his "default Techum"). In contrast, when he is standing outside of a city in an uninhabited area, there is no place of Shevisah that he has in mind other than the place he designates. Consequently, if, for whatever reason, he does not acquire the Makom Shevisah that he had in mind, he has no Makom Shevisah at all.