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within a multistable system, subsystem adapts to subsystem in exactly the same way as an animal adapts to its environment. (l) The environment is assumed to consist of large numbers of subsystems that have many states of equilibrium. The environment is thus assumed to be polystable. (2) Whether because the primary joins between the subsystems are few, or because equilibria in the subsystem are common, the interaction between subsystems is assumed to be weak. (3) The organism coupled to this environment will adapt by the basic method of ultrastability, i.e., by providing second-order feedbacks that veto all states of equilibrium except those that leave each essential variable within its proper limits. (4) The organism's reacting part is itself divided into subsystems between which there is no direct connection. Each subsystem is assumed to have its own essential variables and second order feedback. To trace the behavior of the multistable system, suppose that we are observing two of the subsystems, e.g., A and B and that their main variables are directly linked so that changes of either immediately affect the other, and that for some reason all the other subsystems are inactive. The first point to notice is that, as the other subsystems are inactive, their presence may be ignored; for they become like the 'background'. Even some are active, they can still be ignored if the two observed subsystems are separated from them by a wall of inactive subsystems. The next point to notice is that the two subsystems, regarded as a unit, form a whole which is ultrastable. This whole will therefore proceed, through the usual series of events, to a terminal pattern of behavior. If, however, we regard the same series of events as occurring, not within one ultrastable whole, but as interactions between a minor environment and a minor organism, each of two subsystems, then we shall observe behaviors homologous with those observed when interaction occurs between 'organism' and 'environment'. Trial and error will appear to be used; and, when the process is completed, the activities of the two parts will show co-ordination to the common end of maintaining the essential variables of the double system within their proper limits. Exactly the same principle governs the interactions between three subsystems. If the three are in continuous interaction, they form a single ultrastable system which will have the usual properties. As illustration, we can take the interesting case in which two of them, A and C say, while having no immediate connection with each other, are joined to an intervening system B, intermittently but not simultaneously. Suppose B interacts first with A: by their ultrastability they will arrive at a terminal pattern of behavior. Next B and C interact. If B's step-mechanisms, together with those of C, give a stable pattern of behavior to the main variables of B and C, then that set of B's step-mechanism values will persist indefinitely; for when B rejoins A the original stable pattern of behavior will be re-formed. But if B's set with C's does not give stability, then it will be changed to another set. It follows that B's step-mechanisms will stop changing when, and only when, they have a set of values which forms fields stable with both A and C. (Ashby, l960, pp. 208-2l0)

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