If we want to Prioritise abstraction over metaphor, the concept of a System Boundary needs to be stripped of its metaphorical content. The concept of a Markov blanket delivers exactly that:

[T]]he notion of a Markov blanket allows one to define any System or structure in a way that distinguishes it from the Environment or milieu in which it resides. The Markov blanket plays the role of a statistical boundary that allows one to talk about a system per se.

A Markov blanket is a set of states that separates the internal or intrinsic states of a structure from extrinsic or external states. Importantly, when interactions between states are spatially dependent, as is the case for states pertaining to the physical description of biological organisms, this separation can be spatial in nature. Consequently, in this setting, a Markov blanket describes a spatial boundary.Palacios et al. (2020), 1

Because
The
world is a hierarchy of systems, we can describe it as being
structured by “Markov blankets all the way down”Friston (2019), 7

.

If we “carve up” reality at system boundaries, and these boundaries
are objectively defined as Markov blankets, this carving up has an
ontological character – it is about systems themselves, not just
Models of them.
This is a stronger claim than the one that systems are epistemologically
defined= “real patterns”.Dennett (1991). Cf.
Causal
Emergence for more on this. The ontological character allows us to
objectively identify
Missing
Systems.

## References

- Dennett (1991): “Real Patterns”
- Friston (2019): “A free energy principle for a particular physics”
- Palacios et al. (2020): “On Markov blankets and hierarchical self-organisation”