This identification intervenes from the moment when Descartes determines natural light (which in its undetermined source should be valid even for the mad), from the moment when he pulls himself out of madness by determining natural light through a series of principles and axioms (axiom of causality according to which there must be at least as much reality in the cause as in the effect; then, after this axiom permits the proof of the existence of God, the axioms that the light of nature teaches us that fraud and deception necessarily proceed from some defect.