1) SUMMARY: "KOL SHE'EINO BA'ZEH ACHAR ZEH..."
QUESTION: The Mishnah (49b) 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. Rabah explains that Rav's ruling is based on the principle, "Kol she'Eino ba'Zeh Achar Zeh, Afilu b'Vas Achas Eino" -- any two acts that cannot take effect when done consecutively also cannot take effect when done simultaneously. Just as one cannot make a second Makom Shevisah of four Amos if one already made a Makom Shevisah in another place, he cannot make his Makom Shevisah underneath the entire breadth of the tree (for he is attempting to make two places of Shevisah (of four Amos each) simultaneously).
The Gemara explains that this principle does not apply to Ma'aser. When one attempts to separate 20 percent (instead of the required 10 percent), 10 percent still becomes Ma'aser, even though one cannot separate an additional 10 percent after he has already separated 10 percent. The Gemara explains the reason for this as follows: Since Ma'aser can be separated from half of each grain, when one separates 20 percent it is considered as though he intends to separate one out of every two grains that he designated as Ma'aser (and thus half of the 20 percent, or 10 percent, is Ma'aser).
How is the concept of "Kol she'Eino ba'Zeh Achar Zeh..." related to the Mishnah? The person who makes his Eruv Techumin does not want all eight Amos underneath the tree to be his Makom Shevisah; he wants only four of those eight Amos to be his Makom Shevisah! How can his action be called an attempt to make two places of Shevisah "b'Vas Achas," when he intends to make only one Makom Shevisah?
Furthermore, what is the Gemara's logic when it says that since one can separate Ma'aser by separating halves of grains, the concept of "Kol she'Eino ba'Zeh Achar Zeh..." does not apply?
ANSWER: When one does not specify exactly where his Makom Shevisah should be established but instead designates that four out of a certain eight Amos should be his Makom Shevisah, it is not possible for the "Techum itself" to choose any specific four Amos out of the eight, because there is no criterion by which it should choose. Consequently, the Makom Shevisah automatically "attempts" to take effect on all eight Amos. Moreover, when a person says "four out of eight," as opposed to simply "four," it is clear that he wants some element of Shevisah to exist in all of the eight Amos. (It is not possible to split a Makom Shevisah into two non-contiguous areas.) Since a Makom Shevisah cannot take effect on four Amos after it has already taken effect on a different four Amos, it cannot take effect on eight Amos together at one time.
In the case of Ma'aser, when one separates 20 percent as Ma'aser, he obviously does so because he wants the Ma'aser to take effect on part of all 20 percent -- that is, half of each particle of grain. Since that is possible (Ma'aser can take effect on non-contiguous grains), Ma'aser does take effect.
2) ESTABLISHING "SHEVISAH" UNDERNEATH A TREE
QUESTION: Abaye says that if the ground underneath the branches of the tree is less than twelve Amos across, and a person says that his Makom Shevisah (a four-Amah diameter circle which is considered to be the person's "home" location for Shabbos) is underneath the tree, he is Koneh Shevisah there. Abaye maintains that there is one circle of a four-Amah diameter (i.e. the circle in the center of the less-than-twelve Amos) that overlaps every circle of a four-Amah diameter that can be drawn underneath the tree. Since every four-Amah-diameter circle is partially included in that central circle, the person's declaration is considered to be specific enough so that he is Koneh Shevisah.
Why, though, is the person Koneh Shevisah when there is uncertainty about the location of his Makom Shevisah? Because of this problem, Rav Huna brei d'Rav Yehoshua rejects Abaye's opinion and asserts that the fact that there is one circle of four Amos overlapping every other circle of four Amos is not relevant. Since the exact location of where he wants his specific circle of four Amos to be, he is not Koneh Shevisah anywhere underneath the tree. Rav Huna brei d'Rav Yehoshua concludes that only when the branches of the tree cover a circle which is less than eight Amos across is he considered to have specified a Makom Shevisah. In that case, there is at least one point under the tree (the center of the less-then-eight-Amah circle) that is definitely part of his actual Makom Shevisah.
Rav Huna brei d'Rav Yehoshua's argument is easy to understand. Why does Abaye argue that one is Koneh Shevisah when there is uncertainty, when the location of not a single point of his Makom Shevisah is known?
ANSWER: The RITVA explains Abaye's reasoning as follows. We know that the four-Amah circle centered within the nearly-twelve-Amah circle under the tree must include at least one point of any four-Amah-diameter circle which is drawn under the tree. For this reason, we assume that the center circle is indeed exactly where the person intended to make his Makom Shevisah.