International Encyclopedia of Systems and Cybernetics

W.R. ASHBY emphasizes that stability (as he defined it: i.e. "dynamic stability"):

"… in no way implies fixity or rigidity. It is true that the stable system usually has a state of equilibrium at which it shows no change; but the lack of change is deceptive if it suggest rigidity: if displaced from the state of equilibrium it will show active, perhaps extensive and complex, movements. The stable system is restricted only in that it does not show the unrestricted divergencies of instability"(1960, p.55-6).

This leads him to relate stability to wholeness, in the following way:

"An important feature of a system's stability (or instability) is that it is a property of the whole system and can be assigned to no part of it. The statement may be illustrated by… the practical construction of the thermostat. In order to ensure the stability of the final assembly, the designer must consider:

(1) The effect of the temperature on the diameter of the capsule, i.e. whether a rise in temperature makes the capsule expand or shrink.

(2) Which way an expansion of the capsule moves the lever.

(3) Which way a movement of the lever moves the gas-tap.

(4) Whether a given movement of the gas-tap makes the velocity of gas-flow increase or decrease.

(5) Whether an increase of gas-flow makes the size of the gas-flame increase or decrease.

(6) How a increase in size of the gas-flame will affect the temperature of the capsule.

Some of the answers are obvious, but they must none-the-Iess be included. When the six answers are known, the designer can ensure stability only by arranging the components (chiefly by manipulating (2), (3) and (5) so that as a whole they form an appropriate combination. Thus five of the effects may be decided, yet the stability will still depend on how the sixth is related to them. The stability belongs only to the combination; it cannot be related to the parts considered separatety" (1960, p.56).

and: "The fact that the stability of a system is a property of the system as a whole is related to the fact that the presence of stability always implies some co-ordination of the actions between the parts. In the thermostat the necessity for co-ordination is clear, for if the components were assembled at random there would be only an even chance that the assembly would be stable. But as the system and the feedbacks become more complex, so does the achievement of stability become more difficult and the likelihood of instability greater" (1960, p.57).

One may wonder about the implications of this last sentence for the contemporary human systems (natural and artificial), which become evermore complex.