So far we have been discussing the restriction on transferring objects between public and private domains on Shabbat. Now we get into a finer grained question: What makes something a public domain?
The answer is revealed by a statement of Rav:
One who carries an object four cubits from place to place in a covered public domain — even though transferring an object four cubits in the public domain is prohibited by Torah law — in this case, he is not liable. The reason is that the covered public domain is not similar to the banners in the desert.
Rav knows that just as transferring an object between public and private domains is not permissible on Shabbat, so too carrying something four cubits (about six feet) in a public domain on Shabbat is not permissible. But maybe carrying something four cubits in a covered public domain (like a public tent) is permissible. A covered public domain, he argues, is not a true public domain.
Where does he get this idea? Rav refers back to the encampments the Israelites established in the desert en route from Egypt to the Holy Land. Since the public areas of the encampment were not covered in the desert, he concludes that a public domain is only public if it is uncovered.
The rabbis want us to understand that our observance of Shabbat is not a new invention, but rather takes us back to our beginnings as a nation wandering in the wilderness. (Indeed, as we will explore in greater detail further on, the rabbis repeatedly return to the story of the wandering Israelites to understand what labors are forbidden on Shabbat.) If an action resembles the work that our ancestors abstained from on Shabbat thousands of years ago, then it is prohibited today by Torah law. If not, it is either permitted or only prohibited as a rabbinic protection, as we saw yesterday. Just as Shabbat brings people together today in time and place, the laws of Shabbat unify us across the span of geography and generations.