I apologize in advance if this is too trivial, but... Time and again, I hear something along the lines of "Plank's constant is a unit of action", or "Plank's constant is a unit of action in the old quantum theory". In addition, many texts imply some sort of connection between quantization of action and the phase integral. For example, the Eisberg-Resnik textbook (2 ed, p. 112) has this to say about the Wilson-Sommerfeld rule: "The quantity Integral (p dx) is sometimes called a phase integral, in classical physics it is the integral of he dynamical quantity called the action over one oscillation of the motion. Hence, the Plank energy quantization is equivalent to the quantization of action". Now, as far as I understand Classical Mechanics, the relevant formula for action says dS = pdq - Hdt where S is action, p and q are the generalized momentum and coordinate, and H is the Hamiltonian. Then if we require that the phase integral is quantized according to Wilson-Sommerfeld rule, Integral (pdx) = nh, then it only follows that the Plank energy quantization is equivalent to the quantization of the quantity S + Integral (Hdt). How does one derive quantization of S from here?