The principle of least action δS = 0 has governed physics for 280 years. Every fundamental equation of motion — Newton, Maxwell, Einstein, Schrödinger, Dirac — can be derived from a Lagrangian. Why? This paper derives the Lagrangian from the axiom σ = 1/(1+σ). The Lagrangian L = T − V is the instantaneous imbalance between the outward phase (T = escape fraction = 1/φ) and the inward phase (V = capture fraction = 1/φ²) of a self-referential system. The action S = ∫L dt accumulates this imbalance. The equations of motion δS = 0 select paths of stationary imbalance — the paths that cost the self-referential system the least to maintain.