Rif Eruvin 14a {Eruvin 49b continues ... 50b}

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14a
{Eruvin 49b continues}
What is means by "he said nothing" {in the Mishna, when he said that his Shabbat abode should be under a certain tree or fence that he knows of}?
Rav said: His statement is of no avail whatsoever, such that he may not even proceed to the space under the tree. {Since he has made no statement establishing his Shabbat abode, and so he does not have the tree and does not have the space where he stands.}
And Shmuel said that his statement is of no avail in terms of proceeding to his house {from that tree/fence}, but to the space under the tree he may proceed. And the space under the tree is a donkey driver/camel driver {such that he is pulled in two directions}, such that {if he wants to reach his house}, if he wishes to measure from the north {side of the tree/branches to achieve the <2000 cubits to his house}, he must measure from the south. If he wishes to measure from the south, he must measure from the north. {All this since he did not specify an exact place.}

{Eruvin 50a}
Rava {our gemara: Rabba} said: What is Rav's reason? He did not specify an exact spot.
Some say: Rava {our gemara: Rabba} said: What is Rav's reason? Anything that cannot be acquired in succession cannot be acquired even simultaneously.

To explain {the latter version}: Just as a man cannot acquire four cubits as a place of abode after he already acquired in another place, so too he cannot acquire simultaneously an area equal to two places.

{Eruvin 50b}
Rav Huna the son of Rav Yehoshua said: They only said this about a tree which has under it 8 cubits, but an area which does not have under it 8 cubits, but rather 7 cubits, behold at least a part of his house is well marked out {=the middle square cubit must be part of any 4x4 area marked out}.
That is to say that half of it will only be 3 and 1/2 cubits, and from any location you measure, you will not find 4 consecutive cubits unless you measure 1/2 a cubit from the other section. Thus that 1/2 cubit is established and marked out, and this is a part of his "house."

A brayta in accordance with Rav: One who is travelling on the road and fears lest darkness falls, and he knows of a tree or fence and says "my Shabbat abode is under it," he has said nothing. "My Shabbat abode is place X {whose limits are properly defined}," he walks until place X. When he reached place X, he may traverse its entirety, and outside of it 2000 cubits. When are the words said? By a well defined spot, e.g., he establishes his rest in a mound which is 10 handbreadths high, and has an area ranging from 4 cubits until a bet satayim {= the amount of space required to grow 2 seah of grain}. And so too a valley which is 10 handbreadths deep, and has an area ranging from 4 cubits until a bet satayim. But a place which is not well-defined, he only has 4 cubits. If there were two people, and one knows {of a tree or place} and the other does not know, the one who does not know may give over the establishing of his Shabbat abode to the one who does know.