+Ashby's Law of Requisite Variety
Ashby’s Law of Requisite Variety: If a system is to be stable, the number of states of its control mechanism must be greater than or equal to the number of states in the system being controlled.
Or, as John Naughton states:
In colloquial terms, Ashby’s Law has come to be understood as a simple proposition: if a system is to be able to deal successfully with the diversity of challenges that its environment produces, then it needs to have a repertoire of responses which is (at least) as nuanced as the problems thrown up by the environment. So a viable system is one that can handle the variability of its environment. Or, as Ashby put it, only variety can absorb variety.