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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 criteria 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 area underneath the tree is less than twelve Amos, and the person says that four of those Amos should be his Makom Shevisah, he is Koneh Shevisah underneath the tree. The reason for this is as follows: Since there is one circle of a four-Amah diameter (i.e. in the center of the twelve-Amah area) that reaches into every other circle of a four-Amah diameter that can be drawn underneath the tree, and there is no possible contiguous four-Amah area that is not partially included in that center circle, the person's declaration is considered to be a specific enough description of the area in order to be Koneh Shevisah.

Rav Huna brei d'Rav Yehoshua rejects this suggestion. He asserts that it is irrelevant that there is one circle of four Amos that reaches every other circle of four Amos. Rather, since it is not known where he actually wanted his particular circle of four Amos to be, he should not be Koneh Shevisah anywhere underneath the tree! Rav Huna brei d'Rav Yehoshua therefore says that only when there are less than eight Amos under the tree is he considered to have specified a clear Makom Shevisah, since in that case it is known for certain that at least the middle Amah must be part of his actual Makom Shevisah.

Rav Huna brei d'Rav Yehoshua's argument makes sense. Why does Abaye argue and say that one is Koneh Shevisah even when there is so much uncertainty and it is not known where even one Amah of his Shevisah is actually located?

ANSWER: The RITVA answers that according to Abaye, since the center circle will definitely include part of any circle of a four-Amah radius under the tree, we therefore assume that the center circle is indeed exactly where he wanted his Makom Shevisah to be.

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