Boundaries include things like a cell membrane, a fence around yard, and a national border; see Part 1. In short, a boundary is going to be something that separates the inside of a living systemfrom the outside of the system. More fundamentally, a living system or organism will be defined as
a) a part of the world, with
b) a subsystem called its boundary which approximately causally separates another subsystem called its viscera from the rest of the world,
where
c) the boundary state decomposes into active and passive features that direct causal influence outward and inward respectively, such that
d) the boundary and viscera together implement a decision-making process that perpetuates these four defining properties.
I disagree with this. This has recently been formalized in Andrew Critch’s «Boundaries» Sequence. E.g.: «Boundaries», Part 3a: Defining boundaries as directed Markov blankets.
Also see: Scott Garrabrant: Boundaries vs Frames