...we see that the integrand in (11) must be of the form e, where F is a function of q T, q 1, q 2, … q m, q t, which remains finite as h tends to zero. Let us now picture one of the intermediate q s, say q k, as varying continuously while the other ones are fixed. Owing to the smallness of h, we shall then in general have F / h varying extremely rapidly. This means that e will vary periodically with a very high frequency about the value zero, as a result of which its integral will be practically zero. The only important part in the domain of integration of q k is thus that for which a comparatively large variation in q k produces only a very small variation in F. This part is the neighbourhood of a point for which F is stationary with respect to small variations in q k. We can apply this argument to each of the variables of integration... and obtain the result that the only important part in the domain of integration is that for which F is stationary for small variations in all intermediate q s.... We see that F has for its classical analogue ∫ T L dt, which is just the action function, which classical mechanics requires to be stationary for small variations in all the intermediate q s. This shows the way in which equation (11) goes over into classical results when h becomes extremely small.