Why the Talmud Draws Imaginary Lines All Around Us—and Over Our Heads
Daf Yomi: Our literary critic ambles over rooftops, ruins, and ships, in search of meaning in Jewish commentary
This week’s Daf Yomi reading brought us to Chapter 9 of Tractate Eruvin, an exceptionally dense section, in which some of the previously established rules about eruvs are seemingly overturned. Until now, it had been my understanding that it was only permitted to carry on Shabbat from one private domain—what the Talmud calls a reshut hayachid—to another when they are joined in an eruv. Thus the different houses adjoining a courtyard, or chatzeir, would create an eruv chatzeirot to enable the residents to carry items through their courtyard from house to house.
But in Eruvin 89a, we learn that things are not so straightforward. Take the example of two attached houses, which share a common wall. Each house counts as a separate reshut hayachid for Shabbat purposes; but what about their roofs, which are shared in common between them? Should we consider the whole roof a single domain, or should we draw an imaginary line to extend the common wall between the houses upward, dividing the roof so that each house’s share constitutes a separate domain? And what if the two houses are not the same size, so that one has a larger share of the roof than another: Does that affect how the reshut hayachid is defined?
According to the first mishnah, there are two Tannaitic opinions on this problem. According to Rabbi Meir, “All adjoining roofs of the town are one domain.” Imagine a row of Manhattan townhouses: According to Meir, if you were up on the roof of your house, it would be all right to carry an item all the way down the row of houses to the end of the block, since the whole row constitutes a single reshut hayachid. But this is only the case, Meir specifies, if all the roofs are at the same elevation—“provided that one roof is not 10 tefachim higher or lower.” If a roof is more than 10 tefachim (about two and a half feet) higher or lower than its neighbor, then it constitutes a separate domain, and you can’t carry onto it.
Meir has previously been described as one of the most brilliant and authoritative Tannaim, but in this case, his ruling does not carry the day. Instead, the halacha follows the majority of Sages, who rule that “each roof is a domain unto itself.” This seems like common sense: A roof is in some sense an extension of the house it covers, and so it ought to be considered a separate area, rather than part of a continuous plane of roofs.
But then Rabbi Shimon adds another wrinkle: “Adjoining roofs … are one domain in regard to utensils that began Shabbat in one of these areas, but not in regard to utensils that began Shabbat in the house.” The same holds true of adjoining yards and unenclosed areas. By this logic, if you had a chair on your roof when Shabbat began, it would be permitted to move it to your neighbor’s roof. And if you had a chair in your house when Shabbat began, it would be OK to bring it up onto your own roof, since your house and your roof constitute a single domain. But you could not bring a chair from your house to your roof, and then from your roof to your neighbor’s roof, since your house and your neighbor’s roof constitute two separate domains. (This is always assuming, remember, that you did not create an eruv; if there was an eruv in effect, everything would be different.)
These various rulings of Meir, Shimon, and the Sages are the starting point for a long and complex argument in the Gemara. It is notable that the Amoraim, the rabbis of the Gemara, are just as interested in understanding the logic of Meir’s position as that of the Sages, even though Meir’s ruling does not have the force of law. In a rabbinic argument, every possibility must be thought through, whether it affects actual practice or not. Thus Abaye bar Avin and Chanina bar Avin (their names seem to suggest they are brothers) wonder why Meir insisted that roofs of different heights count as separate domains.
Abaye—a different, more famous Abaye—overhears their conversation and supplies the answer. Meir’s ruling is an example of a law enacted to protect another law. In a public domain, a reshut harabim, anything that protrudes 10 tefachim high and four tefachim wide—such as a post or a hillock—counts as a separate, private domain, so it is forbidden to place anything on top of it on Shabbat. You can’t, for instance, put down a bundle you are carrying and rearrange it on top of a post, then pick it up again. Meir, Abaye explains, did not want Jews to forget this principle, so he applied the same rule to roofs of different heights, as a reminder that a 10-tefachim difference constitutes a difference in domains. His ruling is “a safeguard against doing the same on an elevation in the reshut harabim.”
Soon the Gemara moves on to another problem, involving the rule of gud asik, that is, “extend and raise up.” As we have seen, the Sages allow us to “extend and raise up” the walls of a house in an imaginary line, to create a partition of the roof above. But there is a disagreement between Shmuel and Rav about how to interpret this rule. Gud asik allows you to partition a common roof area into a series of separate spaces, one for each house. This means you cannot carry from one part of the roof to the other.
But what about carrying within each partition? Rav, who “does not apply gud asik,” holds that the roof area does not really have the status of a private domain. Rather, it is a karmelit, a kind of intermediate zone, in which you can carry for no more than four amot at a time (about six feet). Shmuel, who does apply gud asik, holds the reverse: Each roof constitutes a private domain, so you can carry in it without restriction.
But wait, the Gemara objects. First we hear that the Sages do consider each house’s roof a separate domain—that is, they do apply gud asik. Then we hear that Rav does not apply gud asik. How can this be? This is the kind of disagreement between sources that the Gemara loves to examine and, if possible, resolve, and so it is here. To find out what Rav meant, the Gemara consults the traditional teaching “in the academy of Rav,” which preserved his interpretations. According to this school, the Mishnah itself holds that it is only permitted to carry four amot on a rooftop. When it says “each roof is a domain unto itself,” this means simply that the four-amot radius cannot extend from one domain to the other. That is, “one may not carry an object two amot on this roof and two amot on that roof.” In this way, Rav and the Mishnah can both be held to be correct.
This, one might feel, is complicated enough. But in fact, everything I’ve discussed so far is found in just one daf, Eruvin 89a-b—with room left over for a debate about what happens with adjoining asymmetrical roofs, in which one house’s share is bigger than the other. The pages that follow expand, amplify, digress, and complicate. Can you carry from the roof of a house onto the roof of a ruin? What about a ship—do its walls count as the walls of a house, or are they just partitions to keep out the water? What if you were to take the ship out of the water and turn it upside down—then how would the walls be categorized? What about a pavilion, which has a roof but no walls? What if you wore, rather than carried, a hat or scarf from your house into the courtyard, and left it there, and then someone picked it up and carried into another courtyard—would that be an illegal transfer? By the end of this week’s reading, I was left a little dizzy—a condition that never, it seems, troubled the indefatigable Sages.
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