Eruvin 8B

אבל זה כנגד זה, אימא לא.

but if the two breaches are opposite one another, you might say that it is not considered a private domain even with regard to Shabbat. Rav therefore teaches us that even if the breaches of the courtyard line up with each other, carrying is nonetheless permitted therein.

ולרבה דאמר זה כנגד זה אסור, הא דרב במאי מוקי לה ⁠— בזה שלא כנגד זה, תרתי למה לי?

The Talmud raises a difficulty: And according to Rabba, who said that where the alleyway terminates in a backyard and the breaches are one opposite another, carrying is prohibited, how does he construe Rav’s case? Rav’s ruling must refer to a case where the breaches are not one opposite another, and if so, why do I need two rulings? The essence of this halakha, that the yard is deemed a private domain with regard to Shabbat, was already stated in the Tosefta, so why did Rav need to teach another halakha with regard to the very same issue?

אי מהתם הוה אמינא: הני מילי לזרוק, אבל לטלטל ⁠— אימא לא. קא משמע לן.

The Talmud explains that there is a novelty in Rav’s teaching: If one learned the halakha from there, the Tosefta, alone, I would have said that this ruling that the courtyard is a private domain with regard to Shabbat only applies to the issue of throwing, i.e., that one who throws from the public domain into this courtyard is liable, since it is considered a private domain according to Torah law. But to allow carrying in it like a proper private domain, you might say no, that the rabbis forbade carrying in it, owing to the many people passing through it. Rav therefore teaches us that we are not concerned about this, and that carrying in the yard is permitted, even by rabbinic law.