Hold your arm out and close your eyes. It stays up, and nothing about staying up crosses your attention. Muscle has spring in it, tendon has spring in it, and that mechanical impedance acts in zero time, before any nerve reports anything, because it is not a signal path at all. Robotics has a clean way to ask how much that is worth: take the controller away entirely and see what the mechanics do alone. A cart-pole is the standard unstable body, and ours carries a lumped spring and damper at the pole's pivot, the preflex analogue. Gravity pulls the pole over with a torque that grows with angle; the spring pulls it back the same way. So there should be a threshold: below some stiffness gravity wins and the pole falls, above it the spring wins and the pole is passively stable, motor unplugged, no sensor, no feedback, nothing computed anywhere. The formula for the threshold on a free cart is 0.5264 newton metres per radian. The bench in this lesson measures it.