Literary critic Adam Kirsch is reading a page of Talmud a day, along with Jews around the world.Last week, Daf Yomi readers said goodbye to Tractate Shabbat—but not to Shabbat itself. For the next tractate in the cycle, Eruvin, is dedicated to one subcategory of legal questions that arise on Shabbat. Generally speaking, Talmudic law divides space into two main zones, the public domain (reshut harabah)—such as a street—and the private domain (reshut hayachid)—such as a house. (There are also other zones, which will come up later in the tractate.) On Shabbat, it is forbidden to carry an item from one private domain to another, or from the private to the public domain, or more than four amot, or cubits, within the public domain. These general rules give rise to a thicket of questions and problems, and a whole range of legal strategies for circumventing them, which make up the substance of Tractate Eruvin.The title Eruvin literally means “mergers,” but these are not, of course, the kinds of mergers we’re familiar with from Wall Street. Rather, an eruv is a legal merger of different spaces for the purposes of Shabbat, which is designed to extend the range allowed for walking and carrying. (Many American cities have such eruvs in place, including Manhattan.) As I began to discover this week, making sense of these rules requires a better spatial and geometric imagination than I possess; so I’m grateful for the detailed diagrams that the Schottenstein Edition includes in its footnotes.When I first heard about eruvs as a teenager, I regarded them a little indignantly. If the point of Shabbat is to rest from labor, to rejoice in prohibitions, why invent legal fictions to allow you to get around those prohibitions? Isn’t the eruv contrary to the very spirit of Shabbat? Now, after being immersed in the Talmud for half a year, I’ve begun to understand the more flexible imagination that the rabbis brought to these problems. For them, it was not a question of enjoying a holiday from reality, but of finding a way to obey very specific divine laws while still carrying out the necessary business of life. Precisely because they take the letter of the law seriously, they want to know the exact boundaries of that law—what it allows and disallows, and how to tell the difference.Chapter One of Tractate Eruvin addresses a specific subset of space-related questions. In Talmudic times, residential houses were usually grouped around a courtyard (chatzeir), and courtyards connected with the street by means of an alleyway (mavoi). Naturally, it would be very convenient to be able to carry items within the alley. But is the mavoi a private or a public domain? The answer depends on whether it is open at both ends or only one. If a mavoi is closed at one end, it can be marked as a private domain by putting a symbolic closure at the open end—not an actual door or gate, but some architectural feature to distinguish the mavoi from the public street beyond. The rabbis describe three such markers: a sidepost (lechi) that partially closes off the open end; a crossbeam (korah) laid across the top of the walls of the entryway; or a set of posts topped with a crossbar, in the shape of a door (tzurat hapetach).This week’s reading was devoted to the technical requirements for these kinds of markers. The first mishna quoted in Eruvin 2a, for instance, holds that a crossbeam placed over the entrance to an alleyway cannot be more than 20 amot or cubits from the ground. An amah is approximately 20 inches, so the height restriction is about 33 feet. Typically, there is some rabbinic debate about how to define an amah and a tefach or handsbreadth (about 3 inches), and how many tefachim go into an amah. In the course of this digression, the rabbis say that the definition of these measurements was given by God to Moses on Mount Sinai.I found this a revealing moment, since it shows what kind of intellectual work the idea of the Oral Law performed for the rabbis. The Torah speaks of cubits and handsbreadths, but after all, how are we to know exactly how to measure these things? The Torah, like any law, presupposes a world and a language in which that law is practiced. But between the giving of the Torah and the writing of the Talmud, something like 1,500 years had passed, and what was common knowledge to the ancient Israelites was a matter of guesswork to the Babylonian Jews. The Oral Law is what allows Jews to be certain that they are interpreting the Written Law correctly; it collapses the distance between the present and Sinai. To use the language of American constitutional debates, the Oral Law allows the Jews to be confident originalists. For this reason, if the Oral Law didn’t exist, it would surely have to be invented.Continue reading: Not just one answerThe Talmud gives two complementary reasons for the 20-amot rule, one theoretical and one practical. The theoretical reason goes back to the Temple in Jerusalem, where the entrance to the sanctuary was 20 amot high. This way of connecting Shabbat practice with the Temple reminded me of how, in Tractate Shabbat, the 39 forbidden labors or melachot were derived from the construction of the Tabernacle. Indeed, the Gemara spends some time arguing that the Tabernacle, which the Israelites built in the desert, and the Temple, which Solomon built in Jerusalem, are legally and scripturally connected, so that what applies to one also applies to the other.Pragmatically, too, it’s important for the crossbeam not to be more than 20 amot high. This is because the function of the crossbeam is to remind people that they are passing out of a private domain, the mavoi, and into a public one, the street; and if it is too high up, passerby may not notice it. In Eruvin 3a, the rabbis draw an analogy with the succah, where the same height limitation applies. In a succah, the covering or s’chach may not be more than 20 amot from the ground, because the whole point of the succah is to give the feeling of being in an enclosed structure. If the roof is too high, the succah will fail in this purpose. By this logic, however, a crossbeam can be higher than 20 amot if it has a “prominence” (amaltra)—that is, some feature that makes it especially conspicuous. Examples of such features include “nests”—carved cornices resembling birds’ nests—and cedar poles.What do you do if your crossbeam happens to be more than 20 amot high? You could lower it, of course; but as the rabbis point out, with typical thoroughness, you could also raise the ground under it. (This would not have been too hard to do in a place where the streets were made of dirt and could be dug up at will.) This brings up the question of how deep a raised platform one has to build under the crossbeam, that is, how far back into the mavoi it must extend. Rav Yosef holds that one tefach is enough—that is, about three inches. Abaye, on the other hand, requires four tefachim, about a foot. The difference, the Gemara explains, is that Yosef conceives of the crossbeam as just a reminder, while Abaye imagines it as a full partition. It is a good example of how, in the Talmud, what seem like trivial technical debates—a matter of a few inches—rest on deeper conceptual disagreements.The rabbis go on to consider every possible permutation of alleyways and entrances. What if you have an L-shaped alley, which is open to the street at both ends: Does this constitute one open alley or two closed ones put together? What if the walls of a mavoi have crumbled so there’s a breach in them—how big an opening is required before the wall is considered to be legally void? What if an alley is bounded at one end by the sea or by a garbage dump, features that are not permanent and may change their shape?In every imaginable case, the Talmud offers not just one answer, but a range of answers from different authorities. The variety is overwhelming, and I found it a struggle to keep all the different cases clear in my mind. Simply by thinking about the range of problems that might come up in everyday life, the Talmud demands that the student hone his powers of visualization and geometrical reasoning, as well as his powers of logical deduction. I’m beginning to understand how studying the Talmud alone constituted a whole education for generations of young Jews.***Like this article? Sign up for our Daily Digest to get Tablet Magazine’s new content in your inbox each morning.Adam Kirsch is a poet and literary critic, whose books include The People and the Books: 18 Classics of Jewish Literature.