Because commons typically operate in environments that are hostile to them, commons need to protect themselves from the forces of extraction, co-optation and abuse. They need some form of semi-permeable boundary to enable them to safely and selectively interact with the worlds of capital, markets and the state on their own terms.

So, for example, a coastal fishery functioning as a commons may sell some of its fish to markets, but the goals of earning money and maximizing profit cannot be allowed to become so important that it crowds out commons governance, a focus on household needs (oikonomy), and respect for ecological limits.

The semi-permable membrane around a commons functions much as the blood–brain barrier protects our brain, separating 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 -- while also allowing the selective transport of molecules such as glucose and amino acids that are crucial to neural functions. But -- and this is decisive -- the blood-brain barrier 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.)

# Examples - General Public License for free and open source software. - Transvestment rules that permit outside investors to invest capital in a commons, but not to have any decisionmaking power and only with caps on potential return on investment. - Community-made rules prohibiting market sales of wood, game or mushrooms from commonly held forests. - Creative Commons licenses that allow sharing and copying, but only for noncommercial purposes or if the derivative work is also required to be shared (via CC ShareAlike license).