Eruvin 50A

אמר רבה: מאי טעמא דרב ⁠— משום דלא מסיים אתריה.

Rabba said: What is the reason for Rav’s statement that one who declares his intention to establish residence beneath a tree has said nothing at all? It is because the place he designated is not precisely defined. Since he did not establish his residence in one particular location, he did not establish it at all.

ואיכא דאמרי, אמר רבה: מאי טעמא דרב ⁠— משום דקסבר: כל שאינו בזה אחר זה ⁠— אפילו בבת אחת אינו.

And some say an alternative version of Rabba’s statement. Rabba said: What is the reason for the statement of Rav? It is Because he maintains: Anything that cannot be accomplished sequentially, due to halakhic or practical considerations, even simultaneously, cannot be accomplished, as one negates the other. In this case, since one cannot establish residence in an area of 4 cubits on one side of a tree and proceed to establish residence in an area of 4 cubits on the other side of the tree, neither can he simultaneously establish residence beneath a tree greater than 4 cubits.

מאי בינייהו? איכא בינייהו דאמר: ״ליקנו לי בארבע אמות מגו שמונה״.

The Talmud asks: What is the practical difference between these two versions of Rabba’s statement? The Talmud answers: There is a practical difference between them with regard to a case where he said: Let residence be acquired for me in 4 cubits of the 8 or more cubits beneath that tree.

מאן דאמר משום דלא מסיים אתריה ⁠— הא לא מסיים אתריה.

According to the one who said that it is because the place he designated is not precisely defined, here too, the place he designated is not precisely defined, as he failed to specify the precise location of the 4 cubits in which to establish his residence.