The “principle of requisite variety” developed by Ross Ashby, states that in order to be “regulator” of a system you need an equal amount of variety as the system1. Davies draws the analogy to control systems in vehicles a train only has one direction (along the track) and so it requires only a control system to go forward or stop (a lever). A car can go in many directions along the ground (more variety) and so it requires a wheel as well as an accelerator. A helicopter can go up/down, left/right, faster/slower and so it needs a joy stick.
Another version would be that as a system becomes more complex, the regulator (controller) must becoming equally as complex.
1. Davies, D. The Unaccountability Machine: Why Big Systems Make Terrible Decisions – and How the World Lost Its Mind. (Profile Books Ltd, 2024).