Eruvin 34A

אמאי? נימא: כיון דאי בעי אמטויי מצי ממטי ליה, אף על גב דלא אמטייה ⁠— כמאן דאמטייה דמי!

Why must one actually bring the eiruv to the place where he wishes to establish his residence? Let us say: Since if he wished to bring the eiruv there he could bring it, even though he did not bring it, it is considered as though he did bring it there. This follows the same reasoning proposed by Rav Yirmeya in the case of the basket: Since one can tilt it. The fact that this reasoning is not employed here indicates that the potential to do something is insufficient; rather, the deed must actually be done.

אמר רבי זירא: גזירה משום יום טוב שחל להיות אחר שבת.

R' Zeira said: The fact that one must bring his eiruv the day before to the spot that he wishes to establish as his place of residence, and the potential to bring it there does not suffice, is a decree due to a Festival that occurs after Shabbat. In that case, the eiruv is valid for the Festival only if it was brought there before Shabbat, for it cannot be carried there on Shabbat. Since one cannot actually bring the eiruv there, it cannot be said: It is considered as though he did bring it there because had he wished to bring the eiruv he could have. Consequently, the rabbis decreed that in all cases, the eiruv is only valid if it was actually brought to the designated spot, lest one come to think that even on a Festival that occurs after Shabbat it need not be brought there.

איתיביה: נתכוון לשבות ברשות הרבים, והניח עירובו בכותל, למטה מעשרה טפחים ⁠— עירובו עירוב, למעלה מעשרה טפחים ⁠— אין עירובו עירוב. נתכוון לשבות בראש השובך או בראש המגדל, למעלה מעשרה טפחים ⁠— עירובו עירוב. למטה מעשרה טפחים ⁠— אין עירובו עירוב.

Rav bar Shabba raised another objection from a different baraita: With regard to one who intended to establish his Shabbat residence in the public domain and placed his eiruv in a wall that is more than 4 cubits away from that location; if he placed the eiruv below a height of 10 handbreadths above the ground, his eiruv is a valid eiruv; but if he placed it above 10 handbreadths, his eiruv is not a valid eiruv because he is in a public domain while his eiruv is in a private domain. If one intended to establish his Shabbat residence on top of a dovecote or on top of a large cupboard, if he placed the eiruv in the dovecote or cupboard above 10 handbreadths from the ground, his eiruv is a valid eiruv because both he and his eiruv are in a private domain; but if he placed it below 10 handbreadths, the area in which he placed his eiruv is considered a karmelit, and his eiruv is not a valid eiruv because he cannot transport his eiruv from there to his own domain on Shabbat.