The most fundamental concept in cybernetics is that of "difference", either that two things are recognisably different or that one thing has changed … - W. Ross Ashby

Cybernetics treats not things but ways of behaving. It does not ask “what is this thing?” but “what does it do?”... It is thus essentially functional and behaviouristic. Cybernetics deals with all forms of behavior in so far as they are regular, or determinate, or reproducible. The materiality is irrelevant... The truths of cybernetics are not conditional on their being derived from some other branch of science. Cybernetics has its own foundations.

[Constraint] is a relation between two sets, and occurs when the variety that exists under one condition is less than the variety that exists under another.

Its importance is that if R[egulator] is fixed in its channel capacity, the law places an absolute limit to the amount of regulation (or control) that can be achieved by R, no matter how R is re-arranged internally, or how great the opportunity in T. Thus the ecologist, if his capacity as a channel is unchangeable, may be able at best only to achieve a fraction of what he would like to do. This fraction may be disposed in various ways —he may decide to control outbreaks rather than extensions, or virus infections rather than bacillary — but the quantity of control that he can exert is still bounded. So too the economist may have to decide to what aspect he shall devote his powers, and the psychotherapist may have to decide what symptoms shall be neglected and what controlled.