Ideas which have been developed simultaneously or in immediate succession in the same mind mutually reproduce each other, and do this with greater ease in the direction of the original succession and with a certainty proportional to the frequency with which they were together.

p. 90; Cited in: Granville Stanley Hall et al. The American Journal of Psychology, Vol. 35, 1924, p. 218. - Memory: A contribution to experimental psychology, 1885