פירוש על עירובין 9:4

על גבי שני בתים – and they are on two sides of the public thoroughfare, we carry underneath the upper room, for the mouth of the ceiling from each side descends and closes.

Introduction Our mishnah deals with carrying underneath areas that are considered to be public domains.

וכן גשרים המפולשים (bridges under which there is an open passage) – and they have partitions from below on the two sides.

One who builds an upper room on the top of two houses, and so too in the case of viaducts, they may carry underneath them on Shabbat, the words of Rabbi Judah. But the sages prohibit this. There are two situations described in this section. In the first a person builds an upper chamber on top of two houses, meaning it spans two houses which are on opposite sides of the public domain. This means that people below are passing through a “covered public domain”. Similarly, a viaduct is a bridge which passes over the public domain, such that the people who walk beneath it are passing through a covered public domain. Rabbi Judah says that one can carry in a covered public domain, and therefore one can carry in these areas. According to the Talmud, we fictionally draw the edges of the roofs down to the ground so that the public domain has four walls, making it in essence a private domain. The rabbis prohibit carrying in this type of area. Although they do not consider the area to truly be a public domain, and one who carries there is not liable for a Toraitic transgression, it is nevertheless prohibited.

מערבין למבוי המפולש (for an alleyway which is a thoroughfare) – for since there are two partitions, that he (i.e., Rabbi Yehuda) holds that an alleyway that has two partitions, according to the [laws of the] Torah is a private domain. But the Halakha is not according to Rabbi Yehuda.

Rabbi Judah moreover said: an eruv may be prepared for an alley that is a thoroughfare; But the sages forbid this. Most of the alleys that we have been discussing up until now have been alleys with only one open end. All agree that one can set up an eruv for this type of an alley. They disagree with regard to an alley which is open on two ends, one which people can use as a thoroughfare. Rabbi Judah says that one may set up an eruv even for an alley with two open ends whereas the rabbis say that this type of alley is like a public domain and it is forbidden to set up an eruv there.