When
one observes certain distance between his potentially damaging objects
and his neighbor's wall or pit, does he have to do it only when the
wall or pit are already there, or is it true even before the wall is
built or the pit is dug out? Abaye says, “Only when it's there.” Rava
says “Even before it's there.”
Question against Rava from a previous ruling: Olive press refuse... must be distanced three hand-breadths from a neighbor's wall,
which implies that the wall must be there! Rava answers: “No, this is
true even before the wall is there, and this ruling is still needed to
teach us that these materials are harmful to a wall.”