Eiruvin 36a, b

In daf 36a, the rabbis discuss whether an eiruv is prohibited if it "questionable".  The eiruv can be questionable because of the timing of its placement, because of its state of purity, or its consecration.  In 36b, the rabbis speak about placing one eiruv in the west and another in the east.  They debate about how we can justify the validity of placing both eiruvin simultaneously.

The rabbis use the concept of chazakah to discuss these issues.  Chazakah describes the continuation of a given state of being.  The rabbis assume that any person or object maintains its status, or chazakah, unless there is a reason for that status to change.  The second is bereira, which is related to chazakah.  Bereira, retroactive designation, allows the rabbis to justify the placement of an eiruv retroactively.

We also learn about the value placed on a Sage over a primary school teacher.  Interestingly, the rabbis discuss how the eiruv allows Jews to welcome 'important' gentiles or Sages into their communities. At the same time, the eiruv allows Jews to escape from unwelcome gentiles who enter their communities.

I wonder how much the concept of eiruv has been maintained because of halachic stringencies.  Perhaps the eiruv has served other functions including protecting Jews from anti-semitism and creating insular communities for Jews.