[1] This distinction of structure by the writings of Charles S. Peirce, one of America's greatest contributors to the philosophy of science.

Some of my colleagues in systems thinking use a four level hierarchy, separating "Mental Models" from the rest of structure which they then call "Systemic Structure." This is very consistent with my view. However, my view permits two different elements: 1. When we see a Mental Model being held, this is observing an event, and 2. We might see trends in Mental Models.