The Fragility of Resiliency

Resilient systems, natural or engineered, do not usually evoke fragility, neither in the layperson’s nor, typically, in the practitioner’s mind. While some natural systems are resilient and not fragile, all engineered systems I am aware of are resilient yet fragile.

How so? The answer lies in that odd term that appears in the title of this blog: antifragility. Explaining this term, that was coined by Nassim Nicholas Taleb in his 2012 book Antifragile, is the reason I wanted to have this article be one of the first I post on this blog. I will get back to antifragility in a moment, but first let’s review what resiliency and, more basically, robustness are.

Robustness is the ability of a system to function in the presence of internal and external variations. A robust system does not, however, learn about or from these external variations. It is indifferent to such variations . And it does not adapt internally to these variations either. Robust systems, despite their name, are brittle —there’s always a limit to how much variation they can handle. With determination, one can always create conditions beyond the robust system’s design envelope that will result in system failure. Drop that high-quality Danuta rocks glass from high enough an altitude, on a hard enough a surface enough times, and it will eventually break. Every time it survives an impact, that rocks glass neither learns from nor adapts to the severity of every subsequent impact.

A resilient system, on the other hand, both learns from and adapts to internal and external changes and variations. Adaptive NeuroControl is a good example of a resilient engineering capability. Taleb’s book has many examples of resilient biological, social and political systems. The immune system is an excellent example of a resilient natural system. Going back to our rocks glass, it would be resilient, not just robust, if with every impact it learns something about its abilities and limitations and adapts accordingly to improve its chances at surviving the next impact.

Yet, there will always be a limit. A resilient system, if it is not learning and adapting quickly enough, eventually it will break. However fast our rocks glass is learning and adapting, if it is not doing so rapidly enough, it is bound to meet its unfortunate fate. In this sense, then, even resilient systems have an inherent amount of fragility.

Then we have antifragility. Antifragility is not, however, about the speed of learning and adaptation. An antifragile system, simply, is one that thrives on stressors. An antifragile rocks glass is one that thrives on and seeks stressors. A good socio-biological example is the athlete who is constantly seeking a new, untried challenge that help reveal her weaknesses in order to overcome them. She’s learning, adapting and seeking new stressors in order to become more resilient. For engineered systems, this almost necessarily implies autonomy. Learning and adaptation are both needed in any autonomous system. But these two alone do not make an autonomous system antifragile. For the latter, an autonomous system also needs to seek inputs, variations, stressors, etc., that reveal its inherent deficiencies. Nothing is new here, in some sense. It seems to just be how we design our autonomous system’s objective functions. For an autonomous system to be antifragile, the driving force behind its decisions needs to include seeking conditions under-which it learns about its own weaknesses and ignorances about its environment, and then learn from and adapt to what it discovers. Like Taleb puts it, an antifragile package would not have a “Fragile: Handle With Care” label on its box. The label would say, instead, “Antifragile: Please, Mishandle”.

We can thus break down robust, resilient and antifragile systems according to the diagram shown below, where the two main defining parameters are: ability to adapt and kind of reaction to stressors.

I will close with two remarks. The first remark relates to the question of where are space systems, today, on this diagram? In my opinion, they are solidly sitting in the top right corner of the “Robust” block. With programs like DARPA’s Blackjack Pit Boss Program, they are heading towards the resilient. We are still far from the antifragile, but we are heading in that direction.

My second remark is that antifragility has its limits. Like fragile resilient systems, if they are not learning and adapting rapidly enough, or if they seek stressors too aggressively beyond what they can handle, just like the over-zealous athlete, they can break.

In that sense, even antifragile systems can be fragile.

Robustness, Resiliency and Antifragility as a function of adaptation and response to stressors.