If a photographer wants to capture twenty images, and the subject of each image requires a distinct combination of focus and exposure, then the camera must have available at least twenty distinct settings.
This example of requisite variety comes from Ross Ashby‘s 1956 book An Introduction to Cybernetics (pdf download from Principia Cybernetica).
At the core of Ashby’s writing is the question of regulation. How do biological organisms, social organizations, or — in this case — mechanical artifacts regulate for their environments? They must have sufficient diversity or variety. Thus: “Only variety absorbs variety.”
I’ve been reading Ashby with one eye to experiences with climate regulation. From an Ashbian perspective, we might interpret this week’s NYT front-page report on carbon credits functioning as perverse incentives as an example of regulations that have had insufficient variety to cope with the variety of responses. (For reports that have found European climate regulations successful, see “The European Carbon Trading (EU ETS) Experience” and “Insights from the European Union Carbon Market.”)
A few notes from Ashby’s text:
11/2. The subject of regulation is very wide in its applications, covering as it does most of the activities in physiology, sociology, ecology, economics, and much of the activities in almost every branch of science and life. Further, the types of regulator that exist are almost bewildering in their variety. However, we shall be attempting to get at the core of the subject — to find what is common to all.
12/1. Before any regulation can be undertaken or even discussed, we must know what is important and what is wanted. Any particular species has its requirements given — the cat must keep itself dry, the fish must keep itself wet. A servo-mechanism has its aim given by other considerations — one must keep an incubating room hot, another must keep a refrigerating room cold.
13/2. Regulation in biological systems certainly raises difficult problems — that can be admitted freely. But let us be careful, in admitting this, not to attribute the difficulty to the wrong source. Largeness in itself is not the source; it tends to be so regarded partly because its obviousness makes it catch the eye and partly because variations in size tend to be correlated with variations in the source of the real difficulty. What is usually the main cause of difficulty is the variety in the disturbances that must be regulated against.
From management consultant and theorist Gareth Morgan, in 1997’s Images of Organization:
The principle of requisite variety, originally formulated by the English cybernetician W. Ross Ashby, suggest that the internal diversity of any self-regulating system must match the variety and complexity of its environment if it is to deal with the challenges posed by that environment. Or to put the matter slightly differently, any control system must be as varied and complex as the environment being controlled. …
[Requisite variety] suggests that redundancy (variety) should always be built into a system where it is directly needed rather than at a distance. … The principle suggests that when variety and redundancy are built at a local level — at the point of interaction with the environment rather than at several stages removed, as happens under hierarchical design — evolutionary capacities are enhanced.
I’m reminded of the subsidiarity principle or the so-called “clumsy climate strategy,” which includes the principle: Deal with issues at lowest possible level of decision making — nations, provinces, cities.
On Ashby’s law, see also this 1990 talk by fellow cybernetician Stafford Beer: