Rif Eruvin 18a {Eruvin 59b continues ... 60b}

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18a

{Eruvin 59b continues}
With what are we dealing? Where he made a דקה { - at the entrance to the alleyway}. Like this that Rav Iddi bar Avin cited Rav Chisda: If one of the members of the alleyway made a door at the entrance {to his courtyard} he does not invalidate the eruv of the other residents of the alleyway.

To explain דקה - an itztaba {=colonnade}.

Rabbi Zera made an eruv for the entirety of the city of the house {academy} of Rabbi Chiyya, and left out no part {of the city}.
Abaye said to him: For what reason has Master done this?
He said to him {Abaye}: The elders of the city told me that Rabbi Chiyya bar Ashi made an eruv for the entire city and I believed that we may deduce from this that it was originally the city of an individual and because of the public.
He {Abaye} said to him: These same elders told me that {at one time} a refuse heap was there on one side {such that there was only one entrance - and thus it was not the case that it was originally of an individual}. Now the refuse heap has been removed and it is like two entrances and it is forbidden.
He {Rabbi Zera} said to him: I did not know this.

Rav Nachman (bar Yitzchak) cited Rav: A ladder {used to exit the city wall} on one side and an entrance on the other - the ladder has upon it the law of an entrance and the law of a partition. It has the law of a partition, such that {only the other side is reckoned to be an entrance, and thus} we need not leave out {some of the city, but rather one may make an eruv for all of the city}.
We deduce from this that whenever there are two entrances, we may not make an eruv for it {the city} until you leave a bit off, and the reason for this leaving over {some area not covered by the eruv} is so that they will recognize that it is the eruv which permits carrying, and therefore the remnant is left in a forbidden state in order to cause them to remember the eruv.

It {the ladder} has the law of an entrance upon it - by a wall between two courtyards. If they want, they can make a single joint eruv, or if they want they may make two separate eruvs.

{Eruvin 60a}
"Rabbi Shimon says: 3 courtyards of 2 houses each":
Rav Chama bar Guria cited Rav: The halacha is like Rabbi Shimon.

"Rabbi Yitzchak says: Even one house and one courtyard":
Do you really think one house and {separately} one courtyard?! Rather say one house in one courtyard.

Abaye said to Rav Yosef: This of Rabbi Yitzchak, is it a tradition or a logical deduction?
[He said to him: What does this {distinction} matter to us?
He {=Abaye} said to him: Is the gemara to be a mere song?

And the halacha is like Rabbi Yitzchak.

{Note: Rif has a different understanding of the gemara than Tosafot. Tosafot thought Abaye's question was that if it is a tradition based earlier/other Tannaim, it is weightier and should perhaps be accepted, but not so much if it is a mere logical deduction, and further, that Rav Yosef's dismissal of the question was because immediately above, Rav Chama bar Guria cited Rav that the halacha is like Rabbi Shimon, and so this entire discussion is not lehalacha. Still, Abaye still wants to understand in great depth even those parts of Gemara which are not lehalacha. Rif understands that Abaye and Rav Yosef are accepting Rabbi Yitzchak over Rabbi Shimon. Abaye wants to know if it is tradition or logical deduction, and Rav Yosef is saying that either way, we rule in Rabbi Yitzchak's favor. Still, Abaye wants to get to the heart of the matter and see what Rabbi Yitzchak's basis is.}

Mishna:
If a person was in the east, and he said to his son, "Make an eruv for me in the west"; in the west, and he said to his son, "Make an eruv for me in the east", then:
if there are from him to his house 2,000 cubits, and to his eruv more than this, he is permitted to his house and prohibited to his eruv,
to his eruv 2,000 cubits and to his house more than this, he is prohibited to his house and permitted to his eruv.

If a person puts his eruv in the extension of the town {ibura shel ir - in a house within 70 and a fraction cubits of the town}, he did nothing. {because he would have had 2000 cubits from the city limits, including the extension, anyway}

If he put it beyond the bounds, even one amah, {Eruvin 60b} what he gains he loses.

{That is, what he gains in one direction he loses in the other. Initially he could go 2000 cubits in any direction from his town. If he places an eruv 1000 cubits west of town, his Shabbat center is at that eruv. Therefore, if he starts in town, he can travel 3000 cubits to the west: 1000 cubits to his eruv and 2000 cubits past that. However, if he wishes to travel to the east, and starts out in town, he is already 1000 cubits from his eruv, so he may only travel an additional 1000 cubits. Thus, he gained 1000 cubits in one direction but lost it in the other direction.}