Karpaf Redux — Eruvin 67ba
תלמוד בבלי מסכת עירובין דף סז/ב
סלע שבים גבוה עשרה ורוחב ארבעה אין מטלטלין לא מתוכו לים ולא מן הים לתוכו ועד כמה עד בית סאתים הא יתר מבית סאתים מטלטלין אלמא כרמלית היא
From The Contemporary Eruv, translation of Tur §346:
A karpaf [an area that is surrounded by an enclosure and not roofed over, similar in appearance to a courtyard (Rama, Shulchan Aruch, ibid., 346:3)] 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.