1) SQUARING CIRCLES
OPINIONS: The Gemara derives from the dimensions of the non-settled area surrounding the cities of the Leviyim that the 2000-Amah Techum for Shabbos is in the form of a square.
How exactly do we measure the 2000-Amah Techum for Shabbos? Do we draw a square around the city or around the person, with the city or person in the center, such that he actually has more than 2000 Amos to walk in the directions of the diagonals (towards the corners of the square)? Similarly, how do we measure the four Amos in which one may carry in Reshus ha'Rabim (mid'Oraisa), and the four-by-four-Tefach dimensions of a Reshus ha'Yachid?
(a) RASHI says that the area of one's Shabbos Techum is drawn in the shape of a square, with the northern side of the square running from east to west.
With regard to carrying more than four Amos in a Reshus ha'Rabim, the four Amos are measured by drawing a circle around the person with a radius equal to the diagonal of a square of 4 X 4 Amos (or 5 3/5 Amos). Hence, one is not Chayav until he carries a distance of 5 3/5 Amos in a Reshus ha'Rabim.
In order for the top of the pillar to be considered a Reshus ha'Yachid, it must contain diagonals of at least 5 3/5 Tefachim. Therefore, if it is square, an area of 4 x 4 Tefachim qualifies. (If it is circular, it must have a diameter of 5 3/5 Tefachim.) (See also Rashi to Shabbos 8a, DH Gevoha.)
(b) RABEINU TAM says that all of the areas are drawn as circles. Even the Techum for Shabbos is not drawn as a square but as a circle with a 2000-Amah radius. Similarly, it is prohibited to carry in a Reshus ha'Rabim a distance of 5 3/5 Amos (like Rashi). The area of the top of a pillar must contain a circle with a diameter of 5 3/5 Tefachim in order to be considered a Reshus ha'Yachid. A square of 4 x 4 Tefachim does not qualify (not like Rashi).
(c) The RASHBAM says the opposite of Rabeinu Tam. All of the areas are drawn as squares, with one side of the square running east-west. One is liable for carrying in a Reshus ha'Rabim only if he carries beyond a 4 x 4 Amah square which is oriented in such a fashion. To make the top of a pillar a Reshus ha'Yachid, a 4 x 4 Tefach square must fit on top of the pillar while oriented in such a fashion.
2) RASHI'S CONTRADICTORY COMMENTS
QUESTION: Rav Ashi cites the forthcoming Mishnah (52a) as support for Rav Nachman. The Mishnah there discusses a case in which a person leaves his home before Shabbos in order to make an Eruv that will enable him to go to another city on Shabbos, and his friend dissuades him from going and convinces him to return home. Rebbi Yehudah maintains that if the person who returned home had in mind to establish his place of Shabbos rest ("Koneh Shevisah") at the end of his Techum, his Eruv is effective. Rebbi Meir disagrees and maintains that his Eruv is not effective.
RASHI (51b, DH Ihu) writes that according to some Amora'im, the argument between Rebbi Yehudah and Rebbi Meir involves a case in which one expressly stated that he wanted to be Koneh Shevisah in the given place ("Shevisasi b'Makom Ploni"). Others, however, explain that their argument involves a case in which he did not expressly state his intention.
Rashi continues and says that it must be that Rebbi Yehudah and Rebbi Meir argue about a case in which the person did not
state his intention. Rashi cites two proofs that they argue about such a case. First, the Mishnah does not say that he stated his intention. Second, the Mishnah says that Rebbi Meir maintains that the person is a "Chamar Gamal" (that is, he is limited to the area shared by both his original Techum and the Techum he attempted to establish). If the person expressly stated that he wanted to be Koneh Shevisah in a given place (and the question is whether that statement is effective or not), he should either be worse
than a "Chamar Gamal" and lose his Techum entirely, or he should retain his original Techum (see Insights to Eruvin 49:1
Rashi's words are difficult to understand. Rashi first says that the Amora'im argue whether the person stated his intention verbally and said, "Shevisasi b'Makom Ploni." Rashi then proves, in the same paragraph, that it cannot be that the person stated his intention! What is Rashi's intention? Is he attempting to disprove the opinion of one Amora? (MAHARSHA)
(a) The MAHARSHA explains the words of Rashi as follows. When Rashi says that according to some Amora'im, the argument between Rebbi Yehudah and Rebbi Meir involves a case in which "he did not state" his intention, Rashi means that the friend did not actually speak to the person who wanted to make the Eruv to dissuade him. Rather, the person changed his mind on his own. (The Amora to whom Rashi refers is Rav Yosef on 52a, and Rashi's explanation here is consistent with Rashi's first explanation for Rav Yosef's statement there.)
A slight emendation of the text of Rashi here is necessary in order for Rashi's words to conform with the interpretation of the Maharsha. This appears to be the intention of the HAGAHOS HA'BACH (#4).
(b) A novel interpretation for the words of Rashi may be suggested as follows. The text of Rashi in our edition of the Gemara seems to reflect a combination of two different explanations. (In a number of places in Eruvin we find that a later version of Rashi's commentary was printed together with an earlier version, with no words such as "Lishna Acharina" to differentiate the two. See Bibliography to Eruvin, section on Rashi, and see also Rashi to 43a, DH Halachah.) Here, two versions of Rashi's commentary seem to have been printed together as one.
Later (52a), Rashi presents two ways to explain the argument among the Amora'im concerning how to understand the Mishnah. According to Rashi's first explanation, the Amora'im agree that the person did not state his intention and say, "Shevisasi b'Makom Ploni." The second half of Rashi here (51b) conforms with that explanation.
Rashi (on 52a) then cites a second explanation ("Lishna Acharina") which he prefers. According to the second explanation, most of the Amora'im agree that in the case of the Mishnah the person did state, "Shevisasi b'Makom Ploni." One Amora, though, says that there exists one Tana (Rebbi Yosi b'Rebbi Yehudah in a Beraisa) who maintains that one does not have to say, "Shevisasi b'Makom Ploni." It is this second explanation to which Rashi refers in his opening comments here.
When Rashi writes that according to one Amora, the Eruv takes effect even if the person did not state his intention and say "Shevisasi b'Makom Ploni," he refers to Rav Yosef (on 52a) who maintains that according to Rebbi Yosi b'Rebbi Yehudah who argues with Rebbi Yehudah, the person need not state his intention explicitly. (The Gemara here quotes only Rebbi Yehudah and not Rebbi Yosi b'Rebbi Yehudah, because Rebbi Yosi b'Rebbi Yehudah agrees with Rebbi Yehudah with regard to what an Ani or Ashir can do. Accordingly, since that is the subject of discussion here, Rashi is justified when he includes the position of Rebbi Yosi bar Yehudah in his explanation of the Sugya.)
(The DIKDUKEI SOFRIM points out that in the Pizarro edition of the Shas, the words, "u'Plugta" until "Ika l'Man d'Amar d'" (sic) do not appear in the text of Rashi, but appear in a marginal note which explains the Sugya according to the second explanation of Rashi on 52a. In addition, in the Saloniki printing of the Shas, an emendation to Rashi's words says that Rashi later changed his mind and re-explained the Gemara, in his second explanation on 52a, in the way that he rejects here.)
Regarding the questions that Rashi here asks to disprove the second explanation, Rashi himself suggests answers later. First, the reason why the Mishnah does not mention that the person said, "Shevisasi b'Makom Ploni," is because it is no more than a continuation of the earlier Mishnah (49b) which does state that he said these words (Rashi to 52a, DH v'Rav Yosef). Second, Rebbi Meir, who says that the person is a "Chamar Gamal," argues with the previous Sugyos that teach that when there is a doubt about a person's Makom Shevisah, he either loses everything or retains the Techum of his city. Rebbi Meir maintains that when there is a doubt, the person loses only part of the Techum -- the part which is not shared by both the place where he intended to be Koneh Shevisah and the place where he is presently located (Rashi to 52b, DH Ela Afilu).