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MULTISTABLE SYSTEM
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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