Eruvin 35A

חסורי מיחסרא והכי קתני: נתנו במגדל, ונעל בפניו ואבד המפתח ⁠— הרי זה עירוב. במה דברים אמורים? ביום טוב, אבל בשבת ⁠— אין עירובו עירוב. נמצא המפתח, בין בעיר בין בשדה ⁠— אין עירובו עירוב. רבי אליעזר אומר: בעיר ⁠— עירובו עירוב, בשדה ⁠— אין עירובו עירוב.

The Talmud answers: The Mishnah is incomplete and it teaches the following: If one placed the eiruv in a cupboard and locked it, and the key is lost, it is nonetheless a valid eiruv. In what case is this statement said? On a Festival; however, on Shabbat, his eiruv is not a valid eiruv. If the key is found, whether in a city or in a field, his eiruv is not a valid eiruv. R' Eliezer disagrees and says: If it is found in a city, his eiruv is a valid eiruv; but if it is found in a field, his eiruv is not a valid eiruv.

בעיר ⁠— עירובו עירוב, כרבי שמעון דאמר: אחד גגות ואחד חצירות ואחד קרפיפות ⁠— רשות אחת הן לכלים ששבתו בתוכן. בשדה אין עירובו עירוב ⁠— כרבנן.

The Talmud now explains the difference: If the key is found in a city, his eiruv is a valid eiruv in accordance with the opinion of R' Shimon, who said: Roofs, courtyards, and enclosed fields [karpeifot] are all one domain with regard to utensils that began Shabbat in them. Accordingly, a utensil that was left on a roof at the beginning of Shabbat may be carried into a courtyard or a karpef. It is possible to transfer anything located in a city from one place to another in a similar manner. If, however, the key is found in a field, his eiruv is not a valid eiruv, in accordance with the opinion of the Rabbis, due to the prohibition to carry in a karmelit. Although carrying there is merely a shevut, the eiruv is not valid.

רבה ורב יוסף דאמרי תרוייהו: הכא במגדל של עץ עסקינן, דמר סבר: כלי הוא, ואין בנין בכלים ואין סתירה בכלים. ומר סבר: אהל הוא.

The discussion above constitutes one understanding of the Mishnah. Rabba and Rav Yosef, however, both said: Here, we are dealing with a wooden cupboard, and the tanna’im disagree with regard to the following point: The first, anonymous Sage, who rules that the eiruv is valid, holds that the cupboard is a utensil, and that there is no prohibited labor of building utensils, and similarly, there is no dismantling of utensils. Since dismantling a utensil is not included in the prohibited labor of dismantling, one may make a hole in the cupboard in order to access the food used for the eiruv. And the other Sage, R' Eliezer, who invalidates the eiruv, holds that the cupboard is a tent. A wooden implement of such a large size is no longer classified as a utensil; rather, it is considered a building, and therefore it is subject to the prohibitions against building and dismantling on Shabbat.