רש " י ד " ה : קרפיפות - אם של בית סאתים הוא מטלטל בכולו , ואם יותר ולא הוקף לדירה קרי ליה רשות אחת לגבי חבירו , או לגבי חצר , לטלטל שתי אמות בזה ושתי אמות בזה , להוציא ולהכניס מזה לזה :
Rashi on Karpeifos: If its size is a beis se'asayim [5000 square amos] he may carry in all of it, but if it is larger, and not enclosed for [the purpose of] habitation, it is defined as a single domain in respect to another [karpaf] or in respect to a yard, [but only] to carry two amos in the one and two amos in the other and to take out or bring in from one to the other.
The following material is excerpted from The Contemporary Eruv: Eruvin in Modern Metropolitan Areas by Rabbi Yosef Gavriel Bechhofer; it is reprinted here with permission. The first paragraph is a translation of Tur (Orach Chaim #345).
A karpaf1 larger than a beis se’asayim [5000 square amos in any shape] that was not enclosed with the intent to render the area suitable for habitation is, nevertheless, considered a reshus ha'yachid me’d'oraysa. The Sages, however, banned one from carrying an object four amos within such a karpaf, lest one come to carry in a reshus ha'rabbim. Nevertheless, it is permissible to transfer an object from a karpaf to another type of carmelis next to the karpaf, such as to an area encompassing many cultivated fields. This is permitted even though the karpaf is technically a reshus ha'yachid me’d'oraysa [while the area encompassing many cultivated fields is a carmelis me’d'oraysa]. Although the Sages generally forbade transferring objects from a reshus ha'yachid to a carmelis, in this case they allowed such activity, for were they to ban it, people might mistakenly conclude that a karpaf is a reshus ha'yachid even me’d’rabbanan, and would therefore come to carry objects within the karpaf indiscriminately. It was therefore deemed better to permit the relatively uncommon activity of transferring objects from a karpaf to a carmelis - so as to bolster the prohibition of carrying within the karpaf - than to prohibit that activity, lest people then [mistakenly] allow themselves to engage in the far more common activity of carrying objects within the karpaf. This, in turn, could lead people to carry in an actual reshus ha'rabbim. Therefore, if a walled garden larger than a beis se’asayim not designated for habitation adjoins an area encompassing many cultivated fields, it is permissible to take a key from that adjacent area, open the door to the garden, and place the key within the garden.
Several years ago, during a routine tour of an eruv located in a coastal community, it was discovered that the eruv encompassed a large saltwater inlet. This inlet had previously gone unnoticed because a thicket of reeds surrounded it. Examination of other thickets in the area revealed that some of them were in reality marshes. These proved impassable, even by local youth equipped with machetes for the specific purpose of trying to traverse these thickets! The problem that had arisen was the obscure Halacha of “karpaf.”
We have already mentioned the problem of natural walls that are einam mukafim l'dira. An area may also be considered eino mukaf l'dira if it is not an area designated for human habitation. We call such an area a karpaf. If a karpaf that is larger than five thousand square amos (a “beis se'asayim”) is included within an eruv it renders the entire eruv invalid.2 The Poskim extend the definition of human habitation to include any use of the area in question for human needs. This would include parks and any other area suitable for walking or strolling, but not planted fields, unless there are walking paths between the furrows.3 The Rashba, quoted by the Shulchan Aruch, rules that if a body of water included within an eruv is suitable for human consumption or use, even if its water is suitable only for laundry, it does not invalidate the eruv - even if it is larger than five thousand square amos.4 This excludes salty or brackish water. The Dvar Shmuel5 does allow a karpaf consisting of planted fields within an eruv when the significance of the inhabited sector of the enclosure outweighs the significance of the uninhabited, planted sector. Most Poskim conclude, however, that the Dvar Shmuel’s leniency only applies to enclosures consisting of real walls. This ruling is, therefore, generally not relevant in modern urban settings. The Divrei Malkiel,6 and others rule that if an eruv was built around an area that contained a pre-existing karpaf, then the “hekef l'dira” (the act of enclosure for the purpose of enhancing human habitation) of the eruv enclosure counteracts the eino mukaf l'dira of the karpaf. Such an eruv is therefore valid. The case in question in that teshuva, however, concerned an area that was technically suitable for walking, however the gentile owner would not allow anyone to actually do so.
Furthermore, the Divrei Malkiel employs several additional reasons in validating the eruv that included this karpaf. It is therefore difficult to isolate one of his reasons and extrapolate a universal leniency based solely on that one reason.7 The best solution is to exclude a questionable area from the eruv. Constructing a tzuras ha'pesach around the karpaf itself may accomplish this.
An area that is surrounded by an enclosure and not roofed over, similar in appearance to a courtyard (Rama, Shulchan Aruch, ibid., 346:3).
Shulchan Aruch, 358:9. See also Hilchos Eruvin 7:3-15; Yesodei Yeshurun, ibid., pp. 261-265; She'arim Metzuyanim B'Halacha 83:4-6 and the Kuntres Acharon there; Tzitz Eliezer 13:41; and Nesivos Shabbos 12-13. The Halachos of karpaf are amazingly complex. Many Acharonim have written teshuvos on the subject, and their teshuvos are frequently contradictory, as they often vehemently disagree with each other in deciding practical Halacha. A succinct summary of the various methodologies brought to bear in permitting karpeifos within an eruv can be found in Rabbi Kroizer’s essay, No’am, vol. 1, pp. 230-233.
Rabbi Akiva Yosef Kaplan noted that in the source of this Halacha, the Chacham Tzvi, siman 59, cited by the Sha’arei Teshuva, Orach Chaim, 358:8, it seems that it is not necessary that a pathway exist between the furrows, but rather only that it be possible to walk between the furrows. Rabbi Kaplan continued to posit that it seems sensible to assume that there must be a requirement of paths. Otherwise, there practically always exists at least some possibility of walking between the planted rows! Perhaps we might contemplate a “compromise” position: If a pathway exists, that certainly is sufficient. If no path exists, but permission can be secured from the owner of the property to walk at regular intervals through his field in ways that would diminish the “impassable” planted area to an area of less than beis se’asayim, that may nullify the karpaf problem as well. More extensive discussions concerning the necessity and width of the path are to be found in Kesser Ephraim, siman 43; Hilchos Eruvin 7:12 and note 165 there.
Shulchan Aruch, ibid., 358:11. See also Mishna Berura there, 358:85 and the Sha'ar Ha'Tziyun there, 358:80-81. It seems from the Rashba's language that although in our times we rarely use pond or stream water for these purposes, nevertheless, where water is theoretically suitable for those purposes it is not considered a karpaf. This definition still excludes polluted or dirty bodies of water.
Cited in the Biur Halacha 358:9, d.h. Aval Im Nizra. The Chacham Tzvi, siman 59, also cited there by the Biur Halacha, extended the Dvar Shmuel's leniency even to cases where a tzuras ha'pesach preceded the development of a karpaf (such as a planted field) within its perimeter. In the specific case discussed by the Chacham Tzvi, however, other mitigating factors were involved. See also Nesivos Shabbos 13:15 and note 50.
Vol. 4 siman 3.
In Hilchos Eruvin 4:14, note 168, Rabbi Lange notes that the Biur Halacha, ibid., d.h. HaZera'im Mevatlim HaDira would apparently disagree with the Divrei Malkiel. Rabbi Lange therefore says that one may rely on the Divrei Malkiel's heter only “b'dochak gadol.” We have already noted that it is difficult to conceive of a she'as ha'dechak in a normal community setting, see above, note 138.