Each history has associated with it a number, called the amplitude, which defines the probability of that particular path being followed. While the classical path (the dash line) occurs with higher probability, the probability for the other paths vary according to a weighting factor. The probability of going from "here" to "there" is the sum of the probability for all paths. This formulism was originally devised by Richard Feynman for his PhD thesis in early 1940s. Twenty years earlier the transition from classical to quantum had to be formulated with a postulation which can be shown to be equivalent to the method of path integral (see mathematical detail).