Eruvin 60B

מה שנשכר הוא מפסיד.

what he gains in distance through his eiruv on one side of the city he loses on the other side.

גמ׳ קא סלקא דעתך: ״למזרח״, למזרח ביתו. ״למערב״, למערב ביתו.

Talmud: It might enter your mind to say that when the Mishnah states that one was standing to the east, it means that he was standing to the east of his house and that he had instructed his son to establish an eiruv to the west of his house. Similarly, when it states that he was standing to the west, it means that he was positioned to the west of his house and that he had instructed his son to establish an eiruv to the east of his house. In such a case, the person’s house is located between him and his eiruv.

בשלמא ״הימנו ולביתו אלפים אמה ולעירובו יתר מכאן״⁠ — משכחת לה, דמטי לביתיה ולא מטי לעירובו, אלא ״הימנו ולעירובו אלפים אמה ולביתו יתר מכאן״⁠ — היכי משכחת לה?

If so, the question arises: Granted, the Mishnah’s case where there is a distance of 2,000 cubits from his current location to his house, and the distance to his eiruv is greater than this, you can find, as it is possible that he can reach his house without traveling 2,000 cubits and he cannot reach his eiruv. But where do you find a case where there is a distance of 2,000 cubits between him and his eiruv, and the distance to his house is greater than this? The person’s house is located between him and his eiruv.

אמר רבי יצחק: מי סברת ״למזרח״⁠ — למזרח ביתו, ״למערב״⁠ — למערב ביתו?! לא, ״למזרח״⁠ — למזרח בנו, ״למערב״⁠ — למערב בנו.

R' Yitzḥak said: Do you think that to the east means that he was standing to the east of his house, and to the west means that he was standing to the west of his house? No, to the east means to the east of his son, who is depositing his eiruv for him, and to the west means to the west of his son.