But the notion of imaginary walls, as we saw on yesterday’s page, lead back to an earlier conversation (Eruvin 42) about ships.
Recall Rab and Samuel’s disagreement – Rab stating that objects can be moved throughout the ship on Shabbat, Samuel that objects can only be moved within four cubit. Here we have some additional rationale:
‘Rab ruled: It is permissible to move objects about throughout its area’ because it has walls; ‘and Samuel ruled: Objects may be moved only within four cubits’, since the walls were put up for the purpose of keeping out the water.
‘Rab’, explained R. Giddal in the name of R. Hiyya b. Joseph, ‘agrees nevertheless that if (the ship) was turned upside down objects on it may be moved only within four cubits.
Upside down, the ship’s hull becomes a roof!
For what purpose, however, was it inverted? If it be suggested: For the purpose of dwelling under it, why, it could be objected, should its law be different from that of a single roof? — It was inverted rather for the purpose of being coated with pitch.
If it is a kind of ‘dwelling’ (who lives under a boat?) it is a roof. The “walls” rise up to make an imaginary barrier, just like a house. Only this is a “house boat” !