The Mishna (16b) introduces the concept of Lavud, an idea that can be useful not only in making an Eiruv, but in building a Sukka, as well (see Sukka 16b). Lavud means “solid” and it expresses the legal fiction which views separate parts as being united, if the gap between them is less than three tefahim.
According to the Mishna, if three ropes are strung across – with three tefahim between the ground and the bottom one and three tefahim between each rope – and the ropes themselves add up to a width of a tefah, then we view the ropes as a ten-tefah high wall for the purpose of the Eiruv (or a Sukka). The same rule applies to posts that are placed at a distance of three tefahim or less from one-another.
In the Mishna, there is a difference of opinion over where this rule applies.
When the Sages issued this ruling, they spoke exclusively of a caravan; this is the statement of Rabbi Yehuda, who maintains that a partition of this kind, which consists of only horizontal or vertical elements, is permitted exclusively in exigent circumstances. Otherwise, full-fledged partitions are required. However, the Rabbis say: They spoke of a caravan in the mishna only because they spoke in the present, citing the most typical case. Those traveling in caravans were typically unable to erect full-fledged partitions, so they would surround their camps with ropes or boards. However, the in the mishna applies in all cases.
According to Rabbi Yehuda, this type of wall will work only for a Shayara, a caravan of travelers – which was the case discussed in the previous Mishna (15b) – but not for an individual. The Hakhamim believe that the wall is valid under all circumstances, and that the Mishna mentioned the case of the Shayara “because they spoke in the present [tense],” i.e. that they related it to the most common case, but it is not meant to exclude other cases.
While it is common for us to find an argument among the amoraim in the Gemara about whether a case in the Mishna is meant to be limited to a specific situation, it is less common to find such a discussion among the, who were closer to the source of the ruling. Nevertheless, in our case both Rabbi Yehuda and the Hakhamim had the tradition from an early Mishna that the law was taught in the context of travelers – a Shayara. Their disagreement is whether the Mishna meant that case specifically, or merely presented it as a practical example from which no conclusion should be reached.