As we have seen, one may not transport objects from one domain to another. Within a private domain, even in a large house with many rooms, one may move objects around freely, because the entire reshut ha-yaĥid is considered one domain. Moving items within it is not considered transporting them from one domain to another. In contrast, in a reshut ha-rabim, one may move an object within his four amot. Since a reshut ha-rabim is communal, each individual may make use of only the four amot he occupies. Four amot is enough space for one to lie down with arms and legs outstretched. If one moves an item outside his four amot, he has moved it from his space to the communal reshut ha-rabim. This is prohibited by Torah law.
From a spiritual perspective, it should be noted that all the confusion and corruption in the world stems from division and dissension. Nations fight one another, people compete with each other, and ideological movements struggle with one another. Thus, tremendous amounts of energy are wasted on strife and contention. Even an individual is often torn between his seemingly contradictory desires. The way to rectify these tensions is to reveal the unity underlying all existence. With belief that one God created the entire world, we can understand that all the different desires in the world are directed toward one goal. Only by following the Torah’s directions can we harmonize them, and thus improve and perfect the world. Now we can understand why the commandment to “Love your neighbor as yourself” is considered a major Torah principle: it bridges the world’s fissures and helps us reveal its fundamental unity.
We are now in a position to understand why objects may be carried in a reshut ha-yaĥid. In a sense, a reshut ha-yaĥid is a perfected place. The walls that enclose it give it a unity of purpose. Thus all its rooms and the areas within it are considered in one place, within which items may be carried. In contrast, a reshut ha-rabim has yet to be perfected. It still expresses the different interests of many different people, and the objects found within it are not yet considered in one place. This is why carrying anything more than four amot in a reshut ha-rabim is considered a melakha.
As for a karmelit, on the one hand it is not set aside for public use and does not obviously express different interests. Thus, according to Torah law it is the same as a mekom petur, and carrying in it is not prohibited. On the other hand, since a karmelit is used in a variety of ways by a variety of people, it has something in common with a reshut ha-rabim. Therefore, the Sages decided that a karmelit should be treated like a reshut ha-rabim, and carrying in it is prohibited.
If a reshut ha-rabim is enclosed by a wall or fence, and it has gates that are closed at night, its fundamental unity and common denominator is thus revealed, and it is perfected like a reshut ha-yaĥid. One may thus carry throughout. A karmelit can be perfected even without being enclosed by a fence or wall. A tzurat ha-petaĥ alone suffices to make it the equivalent of a reshut ha-yaĥid in which items may be carried (as explained below, 29:2).