Physics Looks for Invariants

One of the deep habits of mind that physics training instills is the search for invariants — what stays constant while everything else changes.

When you perturb a system slightly and watch what happens, you learn more about its structure than you do from observing it at rest. The disturbance reveals the constraints. This is why physicists are so fond of thought experiments, limiting cases, and small perturbations: they expose what the system is actually made of.

This habit of mind is visible throughout Feldenkrais's method. A variation that changes only one thing while holding everything else constant is a perturbation. The way the rest of the movement reorganizes in response tells you something about the system's constraints — what it considers non-negotiable, what it will trade away, what it has been assuming all along.

Differentiation, in this light, is applied physics.