Physical quantities of interest – position, momentum, energy, spin – are represented by observables, which are Hermitian (more precisely, self-adjoint) linear operators acting on the Hilbert space. A quantum state can be an eigenvector of an observable, in which case it is called an eigenstate, and the associated eigenvalue corresponds to the value of the observable in that eigenstate. More generally, a quantum state will be a linear combination of the eigenstates, known as a quantum superposition. When an observable is measured, the result will be one of its eigenvalues with probability given by the Born rule : in the simplest case the eigenvalue λ λ is non-degenerate and the probability is given by | ⟨ ⟨ λ λ → → , ψ ψ ⟩ ⟩ | 2 , where λ λ → → is its associated eigenvector. More generally, the eigenvalue is degenerate and the probability is given by ⟨ ⟨ ψ ψ , P λ λ ψ ψ ⟩ ⟩ , where P λ λ is the projector onto its associated eigenspace. In the continuous case, these formulas give instead the probability density.