Systems exhibiting stability are in a state of balance or equilibrium in which a point representing a measurable value of an aspect of the system remains at rest or moves within set boundaries. Such boundaries may be set arbitrarily within a range, such as the setting of a thermostat, or at a particular threshold, such as maintaining stock in inventory. Essential variables are those which must remain stable to insure survival or maintain the identity of the system. A complex system such as an animal ora human being will have a large number of essential variables which must remain within their limits if the system is to survive and prosper. Stable systems are characterized by their ability to return to a state of equilibrium after a disturbance. They often rely on negative (error correcting) feedback to retain or improve their stability. When the system improves its capacity to remain in or return to a stable state, we may say it has adapted to respond to its disturbances. Whether a system is seen to be stable or not sometimes depends on the scale of time employed.
Weather patterns, for example, appear unstable in the framework of planning an outdoor event but not in the framework of planting a garden. For an environmentalist concerned about the potential of man-made effects such as the ’greenhouse effect', it may be difficult to identify the point where the equilibrium range has been breached because of the normal large variations in weather patterns. Stability is a quality which belongs to the system as a whole. It may result from a combination of subsystems which are either stable or unstable. Within such a system, parts retain veto power over the whole and their activities must be coordinated. Depending on your viewpoint, stability is not necessarily a desirable outcome: e.g. a population of rats infesting a building may learn to avoid traps and poison and maintain its stability in the face of efforts to eliminate them. # SOURCE Ashby, W. R. (1956). Introduction to Cybernetics. London: Meuthen & Company. Ashby, W. R. (1960). Design for a Brain. London: Chapman and Hall. # EXAMPLES • the operation of a hot water heater • maintaining a balance of orders filled to orders received • supply and demand in a perfect market • instances of homeostasis in the human body • the cycle of an undisturbed predator/prey relationship # NON- EXAMPLES • runaway inflation • a high level of population growth • the arms race • a drop in a product's market share # PROBABLE ERROR • Monitoring the stability of the wrong aspect of the system • Fixing one variable which cannot then vary to remain in a stable relationship with related variables • choosing an inappropriate scale of time, distance or speed from which to determine whether or not a system has or has not remained stable • Failure to specify the framework or context in which the stability of a variable is discussed # SEE Homeostasis; Self-Organization; Environment; Ultrastability