Shabbat 8B

נתכוון לשבות ברשות הרבים, והניח עירובו בבור למעלה מעשרה טפחים ⁠— עירובו עירוב. למטה מעשרה טפחים ⁠— אין עירובו עירוב.

One who intended to establish his Shabbat residence in the public domain at a specific site must place food sufficient for two meals for that site to be considered his legal residence. And if he placed the food used for his eiruv in a pit above 10 handbreadths, i.e., less than 10 handbreadths below ground level, his eiruv is an eiruv. If he placed the eiruv below 10 handbreadths from ground level, his eiruv is not an eiruv. Because the pit is a private domain and he may not carry the eiruv from that private domain to a public domain, where he has established his residence, the eiruv is invalid.

היכי דמי? אילימא בבור דאית ביה עשרה, ו״למעלה״⁠ — דדלאי ואותביה, ו״למטה״ דתתאי ואותביה, מה לי למעלה ומה לי למטה. הוא במקום אחד, ועירובו במקום אחר הוא.

The Talmud seeks to clarify the details of this case. What are the exact circumstances? If you say that the baraita is referring to a pit that has 10 handbreadths in depth and the phrase: And he placed it above 10 handbreadths, means that he raised the eiruv and placed it within 10 handbreadths of ground level, and the phrase: Below 10 handbreadths, means that he lowered the eiruv and placed it 10 handbreadths or more below ground level, what difference does it make to me if the eiruv is above 10 handbreadths and what difference does it make to me if it is below 10 handbreadths? In any case, the pit is a private domain, and the principle states that the private domain extends from its lowest point to the sky. There is no difference whether the eiruv was placed higher or lower. In any case, he is in one place, in the public domain, and his eiruv is in another place, in the private domain. Since he cannot take the eiruv out of the pit, his eiruv is not an eiruv.