909. Why Solomon Saw the Need to Enact Eiruvin

Eiruvin 1:5

King Solomon instituted that if a private domain is divided into separate dwelling spaces, each of which is considered private property, and an area is the joint property of all of them, as is the case with a courtyard and its houses, then the common area is treated as a public domain. Similarly, whenever one resident owns private property and treats it as such, it is considered a private domain. It is therefore prohibited to transfer from one's private property to the commonly-owned property the same way it is prohibited to transfer between private and public domains. Each resident must restrict his activities to his own property unless an eiruv has been made even though the entire area is one private domain under Biblical law.

Eiruvin 1:6

An eiruv means that all the residents join together in a collection of food before Shabbos begins. This serves as a statement that they have joined together and they share the food so that none of them has private property. Rather, just as the common area is jointly owned, they likewise all have a share in the privately-owned units, uniting them into one domain. Having to make an eiruv keeps people from mistakenly thinking that it is permitted to transfer between private and public domains.