Public space is the opposite of privat(iz)e(d) space.
The one who privatizes space has obtained that space, bought it or inherited it, seized it or simply occupied it. She/he is protected by laws, habits and power.
The one who privatizes space controls it.
The powerless need the public space.
Public space is not-controlled space.
Public space is the space of powerlessness.
Public space is economically meaningless.
Public space is the space of need.
Public space is the space of transgression.