Eruvin 8A

ולמאי דסליק אדעתין מעיקרא בין שעירבו ובין שלא עירבו פליגי: בעירבו במאי פליגי, בשלא עירבו במאי פליגי?

And with regard to what first entered our minds, that Rav and Shmuel disagree both in the case where the residents of the alleyway and the residents of the yard established an eiruv together, as well as in the case where they did not establish an eiruv together, explanation is necessary. The Talmud seeks to explicate on what point they disagree in the case where they established a joint eiruv, and on what point they disagree in the case where they did not establish a joint eiruv. That is to say, what is the crux of the argument in these two cases?

בשלא עירבו ⁠— פליגי בנראה מבחוץ ושוה מבפנים.

The Talmud explains: In the case where they did not establish a joint eiruv, Rav and Shmuel disagree concerning the halakha governing an alleyway that appears closed from the outside. Outside the alleyway there is a wider courtyard, so that from the perspective of those standing in the courtyard, the breach at the end of the alleyway seems like an entrance, and the alleyway appears to be closed, but appears to be even from the inside. From the perspective of those inside the alleyway, the breach is even with the walls of the alleyway, so that the breach does not look like an entrance, and the alleyway appears to be open. The dispute revolves around the question of whether an alleyway of this kind is considered open or closed. According to the authority who says that it is considered a closed alleyway, one is permitted to carry within an alleyway that terminates in a backyard in this manner.