Eruvin Chapter 10, Mishnah 9. In the second part of Mishnah 4 of our chapter we learned that a person may stand in a private domain and handle objects lying in a public domain, and the reverse, as long as he does not carry them four cubits in the public domain. Our Mishnah brings the opinion of Rabbi Meir, who disagrees and holds that this is not so.
The text of the Mishnah:
"Lo ya'amod adam bireshus hayachid veyiftach bireshus harabim" - a person may not stand in a private domain, reach out his hand into the public domain, take a key lying there and use it to open a door that is in the public domain.
"Bireshus harabim veyiftach bireshus hayachid" - and likewise he may not stand in the public domain and open a door that is in the private domain using a key that is lying in the private domain.
The reason for the prohibition: In Rabbi Meir's view there is reason to be concerned that the person will bring the key over into the domain in which he is standing, and he will thereby be carrying out and carrying in from one domain to another on Shabbos - a prohibition from the Torah.
How, then, can this be permitted? The Mishnah concludes: "Ela im ken asah mechitzah gevohah asarah tefachim" - one who stands in the public domain and wishes to open a door in the private domain with a key that is in the private domain should make a partition ten handbreadths high around himself. Even though until now the place where he stands was a public domain, it has now become a private domain, and the person, the key and the door being opened are all in one domain. This is the opinion of Rabbi Meir, who disagrees with Mishnah 4 of this chapter, from which it emerged that one may handle objects lying in a domain in which the person is not standing.
The Gemara's addition - the law of a karmelis:
The Gemara explains that a word is missing from the wording of the Mishnah, and that Rabbi Meir extends his ruling not only to one who stands in a private domain and opens a door in the public domain and the reverse, but even to one who stands in a private domain and handles an object in a karmelis, or one who stands in a karmelis and handles an object in a private domain. In such a case too it is permitted only by making a partition around the area in the karmelis, so that the whole area becomes a private domain.
The proof that words dealing with a karmelis are indeed missing from the Mishnah comes from the reply of the Sages, who disagree with Rabbi Meir, since their reply was stated regarding a case of a karmelis.
The Sages' reply:
The Sages said to him: "Ma'aseh beshuk shel patamim shehayah biYerushalayim, shehayu no'alim umanichim es hamafte'ach bachalon she'al gabei hapesach" - this was the market in which cattle were fattened for slaughter. They would lock the doors and place the key in the window above the entrance, which is located in the private domain.
It emerges that they were standing in a karmelis - for the markets of Jerusalem had the status of a karmelis, because of the walls that surrounded the city - and they were handling the key in the private domain on Shabbos. From here the Sages prove that at least in a karmelis it is permitted to handle objects, even though the person is standing in a different domain.
Rabbi Yosi cites a slightly different version of the incident: "Shuk shel tzamarim hayah" - it was not a market of fattened cattle but the market in which wool was sold. This makes no difference in practical halachah; it is purely a factual dispute about the details of the incident.
The Gemara relates that Rabbi Meir in fact retracted his opinion regarding the law of a karmelis, and maintained it only regarding the law of a public domain and a private domain, as stated at the beginning of the Mishnah. The Sages who disagree with him, however, as we saw in Mishnah 4, disagreed even regarding the law of a public domain and a private domain, and held that one may handle an object lying in a domain in which the person is not standing, and there is no concern that he might mistakenly transfer the object from a private domain to a public domain or the reverse.
In summary: Our Mishnah brought the opinion of Rabbi Meir, who forbids standing in one domain and opening a door in another domain with a key that is lying there, out of concern for transferring from one domain to another, unless one made a partition ten handbreadths high that unites the domains. The Gemara added that his ruling was stated even regarding a karmelis, and the Sages answered him from the incident in the market in Jerusalem. In practice Rabbi Meir retracted regarding the law of a karmelis, and the Sages permit even in the case of a public domain and a private domain.