By Gil Regev, Ecole Polytechnique Fédérale de Lausanne | gil.regev@epfl.ch
Hollnagel (2006, pp. 17-18) defines the challenge for resilience engineering as the necessity to “understand when a system may lose its dynamic stability and become unstable.” Resilience is therefore the maintenance of dynamic stability. This is all very well, but dynamic stability is also known by another name, homeostasis. Walter B. Cannon coined the term homeostasis in the 1920s, describing it as (Cannon 1929):
“The highly developed living being is an open system [..]. Changes in the surroundings excite reactions in this system, or affect it directly, so that internal disturbances of the system are produced. Such disturbances are normally kept within narrow limits, because automatic adjustments within the system are brought into action, and thereby wide oscillations are prevented and the internal conditions are held fairly constant.
Compare Cannon’s description with Hollnagel’s (2006, p. 16) :
Complex systems must, however, be dynamically stable, or constrained, in the sense that the adjustments do not get out of hand but at all times remain under control. Technically this can be expressed by the concept of damping, which denotes the progressive reduction or suppression of deviations or oscillation in a device or system (over time). A system must obviously be able to respond to changes and challenges, but the responses must not lead the system to lose control.
These descriptions are obviously very close, if not identical. The main aspect of a resilient system, as much as that of a homeostatic system is its ability to keep disturbances within narrow limits, in Cannon’s terms, or to dampen them, in Hollnagel’s terms. Cannon himself used the term resilience to explain homeostasis (Cannon 1939, p. 244):
The possibility of obtaining further insight into the organization which makes for resilience and endurance in spite of the fell blows of circumstance lies in an examination of the ways in which stability is achieved.
Referring to research done by his predecessors, Cannon explains the dynamic nature of this stability. It is therefore surprising that resilience engineers rarely mention homeostasis. One of the reasons may be that homeostasis is understood as the maintenance of a rigid state whereas resilience is seen as bouncing back and dynamic. But this rigidity is not what Cannon had in mind at all. Here is how he explained the choice of the term Homeostasis (Cannon 1929):
Objection might be offered to the use of the term stasis, as implying something set and immobile, a stagnation. Stasis means, however, not only that, but also a condition; it is in this sense that the term is employed. Homeo, the abbreviated form of homoio, is prefixed instead of homo, because the former indicates “like” or “similar” and admits some variation, whereas the latter, meaning the “same,” indicates a fixed and rigid constancy. As in the branch of mechanics called “statics,” the ‘Central concept is that of a steady state produced by the action of forces; homeostatics might therefore be regarded as preferable to homeostasis. The factors which operate in the body to maintain uniformity are often, so peculiarly physiological that any hint of immediate explanation in terms of relatively simple mechanics seems misleading. For these various reasons the term homeostasis was selected.
Notice the painstaking care he takes in choosing a term that does not imply “rigid constancy” or “simple mechanics” to maintain it. Cannon (1929) defined “six tentative propositions” that form the basis of homeostasis. The first proposition is not only important for resilience engineering, it even addresses the subject of this newsletter, i.e. compound problems, or compounded instability (Cannon 1929):
In an open system such as our bodies represent, compounded of unstable material and subjected continually to disturbing conditions, constancy is in itself evidence that agencies are acting, or ready to act, to maintain this constancy.
It is these agencies, acting or ready to act, that are the source of constancy and therefore of dynamic stability and resilience. It is through this constancy of their internal conditions, requiring slight instability (Cannon 1929), that systems maintain their resilience. A simple and obvious example is to examine what is our resilience when our internal conditions are not maintained constant. Our body temperature varies slightly and constantly. But If it drops below or rises above very strict limits (e.g., lower than 34 or higher than 40) we lose most of our resilience.
Homeostasis describes how systems remain more or less the same by limiting the effects of perturbations that can appear in their internal or external environments while needing these perturbations in order to limit them. It provides a precise explanation of what it means to bounce back. Bounce back to what? To the limits that make the system remain more or less the same.
References
Hollnagel, E., Resilience – the Challenge of the Unstable, in Resilience engineering: concepts and precepts, eds Erik Hollnagel, David D. Woods and Nancy Leveson. 2006. Ashgate Publishing Ltd. Kindle Edition.
Cannon, W. B., “Organization for physiological homeostasis,” Physiological Reviews, vol. IX, no. 3, pp. 399–431, 1929. https://doi.org/10.1152/physrev.1929.9.3.399
Cannon, W. B., The Wisdom of the Body. Revised and enlarged edition. New York: Norton & Company, 1939.