Basic Notions – Variety and the Law of Requisite Variety

To control anything, you need a way to measure it — and to control complexity, you need a way to measure how complex a thing actually is. Cybernetics gives us that measure. Ashby calls it variety: “the number of possible distinct states of a system.” The more complex a system, the more states it can be in, and the greater its variety — and the reverse holds too.

From that, Ashby derives his Law of Requisite Variety, which he states with deceptive brevity: “only variety can destroy variety.” Put less cryptically: to control a process that has a certain amount of variety, your control function needs at least that much variety of its own. Or, as it’s sometimes phrased, the larger the range of actions available to a controller, the larger the range of disturbances it can absorb.

The thermostat makes it concrete. Suppose you want a comfortable room. In winter, if the room has only two states that matter to you — comfortable and uncomfortable — then the system you’re controlling has a variety of two, and Ashby’s law says you need a controller with a variety of at least two to match it: warm air on, warm air off. Two states answered by two states. Done.

Now make it harder. What if the comfort you want changes through the day, so you can ease the heating load while the room sits empty? Now the room has more than two states, and a simple on/off switch no longer has the variety to control it. You need a thermostat that can hold different targets at different times — asleep, morning, away, almost home, home. Split weekdays from weekends and the variety climbs again, and the thermostat has to grow more capable still to keep up. The controller must always carry as much variety as the thing it controls. There is no shortcut around it — which is exactly why simple interventions fail on genuinely complex problems.

And human beings are extraordinarily high-variety systems. So the implication of Ashby’s law for a mediator is sobering: to manage the situation, the mediator has to be able to handle the combined variety of at least two whole human beings — and far more once you add the friends, the families, the lawyers. You can see how fast the complexity compounds. We’ll keep following that thread.

References

[1] The terms “control” and “manage” are used interchangeably here.

[2] Ashby, W. R. (1956). An Introduction to Cybernetics (Vol. 37). London: Chapman & Hall.

[3] “Destroy” is Ashby’s own word; the law is also stated as “only variety can absorb variety.”

[4] Heylighen, F., & Joslyn, C. (2001). The Law of Requisite Variety. Principia Cybernetica. http://pespmc1.vub.ac.be/REQVAR.html