Before us is Mishnah 11 in the fourth chapter of Maseches Avodah Zarah. In order to understand the Mishnah properly, according to the Bartenura, we need to learn it together with the Mishnah that follows it, and only afterwards will we be able to arrive at the practical halachah.
The case before us:
A Jew enters into a business partnership with a non-Jew and says to him: you have a winery, but you have no way of producing kosher wine in it; the Jews have no winery, but I will do the harvesting and produce the wine in your facilities. That way we can sell the wine to Jews and turn a handsome profit. This is the arrangement standing in the background of the Mishnah.
"HaMetaher yeino shel oved kochavim" - one who purifies the wine of a non-Jew. The word "purifies" here does not refer to purity in the sense of tumah and taharah, but to the kashrus of the wine: the Jew prepares the wine in the non-Jew's facilities so that it should be like kosher Jewish wine, free of any concern of yayin nesech and stam yeinam.
"V'nosno birshuso b'vayis hapasuach lirshus harabbim" - and he places it in the non-Jew's domain, in a house that opens to the public thoroughfare. That is, on condition that the wine is stored in a room that is exposed to the public thoroughfare, so that people walking in the street can see through its windows what is going on inside.
"B'ir sheyesh bah ovdei kochavim v'Yisraelim" - in a city that has both non-Jews and Jews living in it. In such a situation - it is permitted.
The reason for the leniency: the assumption is that everyone knows about everyone else's business dealings. The Jew's wine is stored in a room whose windows open to the public thoroughfare, and Jews pass by there from time to time and can glance in and see what is happening. Because of this, the non-Jewish owner of the place will not come and handle the wine, since he knows that he is liable to be caught, and he would then lose his good name and his entire business. Therefore we are not concerned that he will go in and pour the wine as a libation or anything of the sort.
The two conditions that emerge so far:
The possibility of supervision - that the wine be stored in a place where the public can see the storage facility.
The existence of supervisors - that there be Jews present in the place who keep an eye on things from time to time and see what is going on there.
According to the Bartenura, a third condition will be added here as well, which emerges from the following Mishnah.
"B'ir shekulah ovdei kochavim":
When there is no possibility of these two conditions being met, such as in a city populated entirely by non-Jews, one cannot rely on the non-Jews to report to the Jews that the owner of the winery has been handling their wine. Therefore it is forbidden, and the wine is not considered kosher for drinking - "ad sheyoshiv shomer" - until he stations a watchman. That is, the Jew who prepares the wine and safeguards its kashrus must station a watchman on site to keep watch over the wine.
"V'ein hashomer tzarich lihyos yoshev umeshamer":
The watchman is not required to sit and guard the wine twenty-four hours a day; rather, even if he comes and goes - it is permitted. It is enough that he comes from time to time to check on the wine, and the non-Jewish owner does not know when he is going to arrive. This awareness alone is enough to prevent the owner from sneaking in and doing something improper, since the watchman is liable to return at any moment, and that would destroy his business and his good name - and people are extremely afraid of losing their reputation.
So it turns out that when there are no Jews in the city who could keep watch, the second option takes its place: appointing a Jewish watchman to check on the wine from time to time.
When the owner of the winery and the owner of the storage facility are not the same person:
Until now we have assumed that the owner of the wine production facility is also the owner of the storage facility. From here the Gemara infers that if Reuven owns the winery, while the kosher wine produced in his winery is stored in the storage facility of Shimon, a different non-Jew, the situation is fine: Reuven will not sneak into Shimon's courtyard to tamper with the wine, lest he be regarded as a swindler and a thief; and Shimon too will not taste the wine, since Reuven is liable to walk in at any moment, and the Jew is likewise liable to walk in at any moment. Therefore, according to the Gemara's inference, everything is conducted honestly.
Rabbi Shimon ben Elazar, however, disagrees and says: "Kol reshus ovdei kochavim achas hi" - all the domains of non-Jews are considered one domain. Rabbi Shimon ben Elazar does not view the two non-Jews as two separate people who keep watch over each other; rather, he is concerned that one hand will wash the other: one of them will sneak in, and the other will look the other way and let him take from the wine or pour it as a libation, receiving some benefit in return. For this reason they cannot be relied upon at all, and their status is that of a single domain.
We have not yet finished clarifying this Mishnah. Only after we learn the next Mishnah will we be able to explain how the two Mishnayos are to be understood according to the Bartenura and Rashi, and, in contrast, according to Rabbeinu Tam, who differs with them. We must therefore postpone the practical conclusion until we now learn Mishnah 12.