SUMMARY: Rav says that if one attempts to make his Makom Shevisah underneath
a tree without specifying in exactly which four Amos his Shevisah will be,
he is not Koneh Shevisah anywhere, not underneath the tree nor in his
present location. Rabah explains that Rav's reason is because of the
principle, "Kol she'Eino ba'Zeh Achar Zeh, Afilu be'Bas Achas Eino" -- two
actions which do not take effect when done consecutively, one after the
other, also do not take effect when done at one time. Since one cannot make
an additional Makom Shevisah of four Amos if one already made a Makom
Shevisah, he also cannot make his Makom Shevisah underneath the tree because
of that principle.
The Gemara explains that Ma'aser, though, is different. When one attempts to
separate 20% (instead of the prescribed 10%), nevertheless 10% becomes
Ma'aser, even though if one first separated 10%, he cannot separate another
10%. The Gemara explains that the reason is because since Ma'aser can be
separated from half of each grain, when one separates 20% it is as if he
intends to separate one out of every two grains that he designated.
What does the concept of "Kol she'Eino ba'Zeh Achar Zeh..." have to do with
our Mishnah? The person making his Eruv Techumin does not want *all* eight
Amos underneath the tree to be his Makom Shevisah; he only wants four of
those eight to be his Makom Shevisah! If so, how can it be called, "be'Bas
Achas," as if he wanted to make *two* Shevisos?
Furthermore, what is the logic of the Gemara 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 where his Makom Shevisah should be 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. Therefore, the Makom Shevisah automatically tries to take effect on
all eight Amos. Moreover, when a person says "four *out of eight*," as
opposed to "four," it is clear that he wanted *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 either.
In the case of Ma'aser, when one separates 20% as Ma'aser, he obviously did
so because he wanted the Ma'aser to take effect on part of *all* 20% -- that
is, half of each particle of grain. Since that *is* possible (i.e., it can
take effect on non-contiguous grains), it does take effect.
QUESTION: Abaye says that if the area underneath the tree is less than 12
Amos, and one says that four of those Amos should be his Makom Shevisah,
then one is Koneh Shevisah underneath the tree. The reason is because since
there is one circle of four Amos diameter (i.e. in the center) that reaches
into every other circle of four Amos diameter that can be drawn underneath
the tree, and there is no possible contiguous four-Amos area which is not
partially included in that center circle, it 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. It is irrelevant if
there is one circle of four Amos that would reach into every other circle of
four Amos. As long as it is not known where indeed he wanted *his*
particular circle of 4 Amos to be, he should not be Koneh Shevisah anywhere
underneath the tree! Therefore, Rav Huna brei d'Rav Yehoshua says that only
if there is less than *8* Amos under the tree is he considered to have
specified a clear Makom Shevisah, since it is known for sure that at least
the middle Amah must be part of his actual Makom Shevisah.
Why indeed did Abaye think that one can be Koneh Shevisah even where there
is so much uncertainty, and it is not known where even one Amah of his
Shevisah actually is?
ANSWER: The RITVA answers that according to Abaye, since the center circle
will definitely include part of any 4-Amah radius circle under the tree, we
therefore *assume* that the center circle is indeed *exactly* where he
wanted his Makom Shevisah to be.