Communitarian goods are defined by non-excludability among group members

Evidence

A good will be common to the extent that it cannot be increased (or decreased) for any member of the relevant group without at the same time being increased (or decreased) for other members of the group: it has the sort of non-excludability that economists ascribe to goods like clean air and external defence. It will be a partially common good to the extent that it cannot be increased for one without being increased for some; it will be a perfectly common good to the extent that it cannot be increased for one without being increased for all. — Philip Pettit, Republicanism, 573

Categorization

• DEFINE: A tight formal definition of social vs. common vs. partially/perfectly common goods, borrowing economists’ non-excludability.