Systems which are able to **return to equilibrium** after disturbances which were not anticipated or foreseen by their designer are said to be ultra (meaning beyond)stable. Such systems may rely on multiple interacting measures or rules of thumb that suggest leaving a situation when it becomes problematical. Ultrastable systems may rely on algedonic signals or step functions to tell them that an important threshold has been breached even if they do not know the source of the problem. Typically such disturbances are infrequent and therefore cannot be predicted in detail. # SOURCE Ashby, R. (1960). Design for a Brain. London: Chapman and Hall. # EXAMPLES • leaving a party when people begin to show signs of intoxication • selling an investment when the balance sheet becomes impenetrable • adapting to a new situation # NON-EXAMPLES • small, frequent disturbances that are anticipated # PROBABLE ERROR • Not recognizing that a novel set of circumstances is occurring • Not listening to the algedonic signal • Denial of evidence that doesn't fit into a preconceived pattern # SEE Algedonic Signal; Homeostasis