**Statement:** Pre-observation, systems exist in superposition of all possible states.
State space: Complex Hilbert space H.
Pure states: Rays in H (equivalence classes of unit vectors).
Superposition principle: If |ψ₁⟩ and |ψ₂⟩ are states, so is α|ψ₁⟩ + β|ψ₂⟩.
This is an axiom of quantum mechanics—not derived, but postulated and confirmed by experiment.