System boundary

The system boundary is the distinction that separates a system from its environment. Unlike a physical membrane, the system boundary is a functional and operational distinction: it is drawn by the system's own operations, not by an external observer. What counts as "inside" depends on what the system is doing and how it maintains its identity. In autopoiesis, the boundary is produced by the system's own processes; in social systems, the boundary is drawn by the system's code — legal/illegal, true/false, payment/non-payment. The boundary is not a given; it is an achievement, and it is always contestable. Every system must continuously reconstitute its boundary or dissolve.

The boundary is where the system meets its environment, but it is also where the system constructs its environment. A system does not respond to its environment; it responds to the perturbations that cross its boundary, and it determines what those perturbations mean. The boundary is therefore not a passive interface but an active filter. See also Operational closure, structural coupling, and the system/environment distinction.