The following is Rav Dovid’s letter:
הנני בזה לפרסם דבר שידוע לרבני השכונה ואין צריך פירסום הנה יותר מעשרים שנה באו רבנים מפלאטבוש ושאלו לאאמו"ר זצ"ל אם אפשרית לעשות עירוב בפלאטבוש ולאחר שנתברר שיש דרים בפלאטבוש וסביבותיה יותר משנים וחצי מילליאן אמר שאי אפשר, וזה היה ידוע כל השנים הללו ועכשיו אחר שכבר הלך לבית מנוחתו רוצים איזה אנשים לזייף הדבר ואומרים שהוא מסכים לעשות עירוב בפלאטבוש וזהו שקר מוחלט כי שמעתי מפי קדשו שאסור ולא עד מפי עד ומבקש אני מאנשים אלו שלמען האמת לא יוסיפו לשתפו לשיטתם.
ועל זה באתי על החתום ביום ג' לפ' שבתותי תשמורו שנת תשס"ה
From the above, it is clear that Rav Dovid is referring to his father’s position on the 1978-9 Flatbush eruv and not to his father theoretical position on the current eruv. The question is why would Rav Dovid feel a need to clarify his father’s view on the past eruv if the issue is about the current eruv? The only explanation is that Rav Dovid must have been informed that some had argued that his father would even have allowed an eruv in 1978-9.
However, no one could have claimed that Rav Moshe zt”l would have allowed a Flatbush eruv in 1978-9 — given his understanding of the facts on the ground at that point — since he stated clearly that he was of the opinion that an eruv should not be erected then (Igros Moshe, O.C. 4:87-88, 5:28-29; see more below). In truth, the main argument that has been expressed by those supporting the current eruv is that if Rav Moshe would have known the facts on the ground today, he would have allowed an eruv to be established.
Following this, there is no doubt that just as Rav Moshe zt”l was misled regarding the facts on the ground in 1978-9 Brooklyn (see below), so, too, his son Rav Dovid was misled regarding the actual arguments of those supporting the current Flatbush eruv.
Now let us further explore Rav Dovid’s statement regarding his father zt"l’s reason for objecting to the 1978-9 Flatbush eruv. As detailed previously (see Part 2: The Permissibility of a Brooklyn Eruv According to Rav Moshe Feinstein zt”l), Rav Moshe concurred that if Brooklyn’s population is less than three million, the prohibition against the establishment of an eruv there is only because of his personal gezeirah and not because the matter is a d’Oraysa (ibid., O.C. 5:29; see also 4:88). Consequently, when Rav Dovid stated in his letter that he heard from his father zt”l that since, “there are over 2.5 million people living in Flatbush and its environs … it is impossible to build this Eruv,” Rav Dovid must have been alluding to his father zt”l’s gezeirah and not that his father maintained that the matter is a d’Oraysa. In fact, Rav Dovid stated himself that his father zt”l would allow an eruv if the population of a city is less than three million (West Rogers Park Eruv, 1993 page 23); therefore, according to Rav Dovid, the issue in Flatbush is definitely no longer a matter of a d’Oraysa (since Chicago’s population is similar to Brooklyn, the question is why, according to Rav Dovid, could an eruv have been established there; just like the situation in Brooklyn, his father would have been opposed in Chicago because of his gezeirah). [Furthermore, it’s not clear what Rav Dovid intended with this statement that, “there are over 2.5 million people living in Flatbush and its environs.” According to his father zt”l, the boundaries of a city are not measured as one unit but rather in blocks of twelve mil by twelve mil. Moreover, since Brooklyn encompasses an area of more than twelve mil by twelve mil (Igros Moshe, O.C. 4:87-88), the entire population of Brooklyn would not even be included in the calculation to evaluate if it’s a reshus harabbim.]
It’s important to note, Rav Dovid’s argument that his father zt"l’s objection was regarding the size of the population of Flatbush and its environs addresses only one of the three independently suggested reasons that would make an eruv in Brooklyn permissible even according to his father zt”l. However, Rav Moshe in his teshuvos provided two additional reasons why he personally objected to the 1978-9 Flatbush eruv which was based on information that had been related to him at that time (see this post Hagaon Rav Moshe Feinstein Zt”l’s Reluctance to Pasken Against the Establishment of an Eruv for the reason why I stress the word personally). Rav Moshe argued that the population of Boro Park and Flatbush independently is greater than shishim ribo (ibid., 5:28:5 and Addendum to O.C. 4:89) and that Brooklyn is not encompassed by mechitzos (ibid.). Since the facts on the ground have been otherwise confirmed ― the population of Boro Park and Flatbush is less than shishim ribo (see Part 3: The Permissibility of a Brooklyn Eruv According to Rav Moshe Feinstein zt”l) and the presence of mechitzos encompassing Brooklyn has been verified (see Part 4a: The Permissibility of a Brooklyn Eruv According to Rav Moshe Feinstein zt”l) ― one can extrapolate from Rav Moshe’s teshuvos how he would pasken regarding an eruv today. In light of the current situation, there is no reason to argue that Rav Moshe would object to the present eruv. In fact, Rav Tuvia Goldstein zt”l Rosh Yeshiva of Emek Halacha and a Talmid/Chaver of Rav Moshe Feinstein zt”l, said on numerous occasions that based on the current situation Rav Moshe himself would have allowed an eruv. [Furthermore it must be stressed that since Rav Moshe’s shitos in eruvin are admittedly chiddushim (ibid., 1:139:5) it is unreasonable to create additional stringencies and extend his chiddushim further because one believes that Rav Moshe would always have prohibited an eruv.]