ודרך הרבים ודרך היחיד ושביל הרבים ושביל היחיד הקבוע בין בימות החמה ובין בימות הגשמים
and a public road that is at least sixteen cubits wide; and a private road that is four cubits wide; and a public trail; and a permanent private trail that is used whether in the summer or in the rainy season, i.e., winter. Rav Asi and Ravin disagree with regard to whether Rabbi Yoḥanan held that a boundary or sea squill also serves to subdivide a field for the purpose of pe’a.
טומאה מאי היא דתנן הנכנס לבקעה בימות הגשמים וטומאה בשדה פלונית ואמר הלכתי למקום הלז ואיני יודע אם נכנסתי לאותו מקום ואם לאו ר"א מטהר וחכמים מטמאין
The Gemara further clarifies: What is the halakha of ritual impurity that is affected by determining whether an area is one or two fields? As we learned in a mishna (Teharot 6:5): With regard to one who enters into a valley during the rainy season, i.e., winter, when people generally do not enter this area, and therefore for the purpose of this halakha it is considered a private domain, and there is a principle that in a case of uncertainty concerning whether one contracted ritual impurity in a private domain he is ritually impure; and there was ritual impurity in such and such a field, and he said: I know I walked to that place, i.e., I walked in the valley, but I do not know whether I entered that place where the ritual impurity was or whether I did not enter, Rabbi Eliezer deems him pure and the Rabbis deem him impure.
שהיה ר"א אומר ספק ביאה טהור ספק מגע טומאה טמא
Rabbi Eliezer deems him pure, as Rabbi Eliezer would say: Concerning uncertainty with regard to entry, i.e., it is uncertain whether he entered the area where the ritual impurity is located, he is ritually pure. But if he certainly entered the area where the ritual impurity is located and the uncertainty is with regard to contact with ritual impurity, he is ritually impure. It is with regard to this halakha that Ravin said in the name of Rabbi Yoḥanan that a boundary or sea squill defines these fields as distinct areas.
אבל לשבת לא
The Gemara infers, though, that even Ravin holds that a boundary or sea squill serves as a barrier only with regard to pe’a and ritual impurity, but with regard to the halakhot of Shabbat, they do not serve as a barrier between fields.
רבא אמר אפילו לענין שבת דתניא הוציא חצי גרוגרת לרשות הרבים והניחה וחזר והוציא חצי גרוגרת אחרת בהעלם אחד חייב בשני העלמות פטור
Rava says: They serve as a barrier between fields even with regard to the matter of Shabbat, as it is taught in a baraita: With regard to one who carried out half of a dried fig from a private domain into the public domain and placed it there, and then returned and carried out another half of a dried fig, if it was done within one lapse of awareness, i.e., he did not remember in the interim that this act is prohibited or that it was Shabbat, the two acts are considered as one, and since the two items together equal the size of a dried fig, he is liable to bring a sin-offering. But if it was done within two lapses of awareness, i.e., after he carried out the first half of a dried fig he remembered that this act is prohibited or that it was Shabbat, but subsequently forgot again and carried out the second half of a dried fig, he is exempt.
רבי יוסי אומר בהעלם אחד
The baraita continues. Rabbi Yosei says: If it was done within one lapse of awareness,