ashbys-law-of-requisite-variety ADG-0003

Ashby's law of requisite variety

“A regulator can only control a system if it has at least as many distinguishable responses as the disturbances it must handle.”

Also: Only variety can absorb variety · Law of requisite complexity

Origin

W. Ross Ashby, 1956, An Introduction to Cybernetics; Stafford Beer's phrasing is "variety absorbs variety". Boisot and McKelvey restated it as the law of requisite complexity. About control systems in general, treated by Ashby as a law of nature rather than a rule of thumb.

Why it should, or should not, apply to models

A harness is a regulator of a model, and the model's failure modes are the disturbances. The law then says what a harness engineer can and cannot do: a fixed set of rules, gates and sensors can only catch the failures it has a response for, and an open-ended model will produce disturbances outside that set. The prediction is that any control which covers fewer paths than the model can take will be bypassed, not by malice, but because the variety is simply there. It also says why the ratchet has to keep turning: each new failure is new variety the regulator must absorb.

Standing: Holds

Standing counts claims a person has reviewed. Claims reviewed by AI alone are listed and marked, and do not move it.

Where it holds

Claims showing the heuristic applying to models.

Gating irreversible actions works only if the harness mediates every side effect, not only the branch awaiting approval. Where a gate suspends its own branch while a sibling's effect still executes, the control reads as a barrier and is not one — and the shipped implementations in widely used agent frameworks have this gap rather than it being a misconfiguration.
Prioritizing safety under conflicting goalsReviewed by AI and Russ Miller
A gate that mediates one branch while a sibling's side effect runs has less variety than the system it regulates, and reads as a barrier without being one. The shipped frameworks have exactly this gap.

Add a case where it holds or breaks

A break is worth more than a hold. If you have seen this fail for a model, with the setup written down, that is the most useful thing you can add here.

Notes

One claim is thin backing for a law this general. Wanted: any measured case where the number of distinct failure modes an agent exhibited was compared with the number of distinct checks its harness ran.