a term developed by Ashby and defined by him as follows: Two systems of continuous variables (that we called 'environment' and 'reacting part') interact, so that a primary
feedback (through complex sensory and motor channels) exists between them. Another feedback, working intermittently and at a much slower order of speed, goes from the environment to certain continuous variables which in their turn affect some step-mechanisms, the effect being that the step-mechanisms change value when and only when these variables pass outside given limits. The step-mechanisms affect the reacting part; by acting as parameters to it, they determine how it shall react to the environment.
We can now appreciate how different an ultrastable system is from a simple system when the conditions allow the difference to show clearly. The difference can best be shown by an example. The automatic pilot is a device which, amongst other actions, keeps the airplane horizontal. It must, therefore, be connected to the ailerons in such a way that when the plane rolls to the right, its output must act on them so as to roll the plane to the left. If properly joined, the whole system is stable and self-correcting: it can now fly safely through turbulent air for, though it will roll frequently, it will always come back to the level. The Homeostat, if joined in this way, would tend to do the same. (Though not well suited, it would, in principle, if given a gyroscope, be able to correct roll.) So far, after a small disturbance; but connect the ailerons in reverse and compare them. The automatic pilot would act, after a small disturbance, to INCREASE the roll and would persist in its wrong action to the very end. The Homeostat, however, would persist in its wrong action only until the increasing deviation made the step-mechanisms start changing. On the occurrence of the first suitable new value, the Homeostat would act to stabilize instead of to overthrow; it would return the plane to the horizontal; and it would then be ordinarily self-correcting for disturbances. There is therefore some justification for the name 'ultrastable'; for if the main variables are assembled so as to make their field unstable, the ultrastable system will change this field till it is stable. The degree of stability shown is therefore of an order higher than that of the system with a single field. (Ashby, l960, pp. 98, l08)
