The final expression is clarified by introducing the variable canonically conjugate to J, which is called the angle variable θ : The derivative with respect to time is a derivative with respect to θ, up to a factor of 2π T, 2 π π T ∫ ∫ 0 T d t ( d P d J d X d θ θ − − d P d θ θ d X d J ) = 1 . So the quantum condition integral is the average value over one cycle of the Poisson bracket of X and P.