In commons unfriendly environments, each commons needs protection from extraction, cooptation or abuse. Given the power-imbalances between the sphere of the commons and the sphere of capital, semi-permeable boundaries protect the commons, just as the blood–brain barrier protects our brain. It is a highly selective semipermeable membrane that separates the circulating blood from the brain fluid in the Central Nervous System. The blood-brain barrier allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion. It also allows the selective transport of molecules such as glucose and amino acids that are crucial to neural function. But, and this is decisive, it prevents the entry of potential neurotoxins by way of an active transport mechanism. Interestingly, a few regions in the brain, including the circumventricular organs, do not have a blood–brain barrier. Imagine the brain as the commons. It needs a similarily effective membrane for protection.
The idea of setting semi-permeable boundaries includes the possibility to constantly renegotiate these boundaries.