From Stanley Salthe: Summary of the Principles of Hierarchy Theory
"In the specification hierarchy the highest relevant level is always the one in focus, with all the
lower levels of the hierarchy providing cumulative initiating conditions simultaneously upon it.
(This reflects the fact that this hierarchy is implicitly developmental, with the levels being viewed
as having emerged consecutively from the lowest, or most general, up -- as with, e.g., biology
emerging from chemistry, both historically and at any given moment.) The two-level form is
unstable, allowing new levels to emerge at the top of the hierarchy. Use of this form provides us a
model of how change has been possible in the world."