A semi-permeable membrane is a key component for coherent emergent systems.

You need to get the concentration right.

Not necessarily high, but right.

You want a system that can gatekeep at the boundary, to only allow certain kinds of things to enter (while also allowing novelty and diversity).

And also want a credible system to zap invaders.

This can create an emergent system with radically different characteristics than its surroundings.

When you have a positive pressure differential for joining you get people who want to be there: who believe.