hierarchy

kinda related to the last one, this is probably a well-worn point by now, but just to remember that "hierarchy" is not that great of a metaphor, it's probably more like heterarchical "message passing" (john holland), where a given structure can be re-used for processing data at different "levels of abstraction". complementing the idea of scale-invariance; like you maybe could use the same circuitry to find "edges" in retinal pixels or in abstract concepts.