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.
Ashby states the Law as:
variety can destroy variety.
He sees this as aiding the study of problems in biology and a "wealth of possible applications" .
He sees his approach as introductory to Shannon Information Theory (1948) which deals with the case of "incessant fluctuations" or noise.
The Requisite Variety condition can be seen as a simple statement of a necessary dynamic equilibrium condition in information theory terms ''cf.'' Newton's third law, Le Chatelier's principle.
A system has good Control if and only if the dependent variables remain the same even when the independent variables or the State Function have changed.In a real system this implies that the State Function is a composition of two functions, such that the second is the inverse of ( the possible changes of ) the first
# See also - A Mathematical Theory of Communication