Beyond Return to Baseline: Reachability, Observability, and the Measurement of Physiological Margin

Perturbation-recovery assessment measures how a system returns from a challenge: how fast, how completely, and how consistently on repetition. Consider two people whose recovery is identical on all three. One retains several partially independent routes back to function; the other has one left. No measurement of the trajectory distinguishes them, and the second will fail on the next perturbation that closes the route it has. What is missing is not precision but a quantity. Rate, completeness and stability each describe a single trajectory back to a single state, and none registers how many ways back remain. A systems-theory review of homeodynamics arrives at the same gap independently, naming as open the need for an indicator that a system is nearing a critical transition while it can still reorganise. Health defined as retained capacity has acquired an operational method, and the method cannot see what most matters about capacity. Taking as given the viability-constrained reachable set already used in control-theoretic treatments of ageing, we argue the quantity worth measuring is not the cost of restoring lost function but margin: depth within that set's interior, read as expected time to leave the viable region. Depth is gated twice: reachability settles which states are accessible in principle, observability which of those the organism can navigate toward using its own sensing. Drift along directions it cannot sense is a distinct, silent failure class. Route diversity - the number of distinct paths by which return remains possible - is proposed as a fourth recovery dimension, measurable within existing protocols; and loss of achieved control is separated from loss of reachable room by asking whether external actuation succeeds where endogenous regulation failed. Six predictions are stated, four decidable with current instruments, with the observations that would falsify them.