Every stable system has the property that if displaced from a state of equilibrium and released, the subsequent movement is so matched to the initial… - W. Ross Ashby

Two main lines are readily distinguished. One already well developed in the hands of von Bertalanffy and his co-workers, takes the world as we find it, examines the various systems that occur in it - zoological, physiological, and so on - and then draws up statements about the regularities that have been observed to hold. This method is essentially empirical. The second method is to start at the other end. Instead of studying first one system, then a second, then a third, and so on, it goes to the other extreme, considers the set of all conceivable systems and then reduces the set to a more reasonable size. This is the method I have recently followed.

[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.

The fundamental questions in regulation and control can be answered only when we are able to consider the broader set of what it might do, when “might” is given some exact specification.