Freud once said, 'I was always open to the ideas of G. T. Fechner and have followed that thinker upon many important points'. Fechner and Freud had certain similarities as thinkers and as personalities, so that it is an open question to what extent similarities in their work are the result of Fechner's influence upon Freud. There are several psychoanalytic concepts in which the influence of Fechner is most certain and most direct. One of these is the concept of mental energy that ultimately derived from Fechner's concept of 'psychophysical energy'. The topographical concept of the mind, too, was derived partly from Fechner's work. Fechner stated: 'The seat of action of dreams is different from that of waking ideational life'. Freud, like Fechner before him, liked to state general principles. Freud saw his principle of constancy as a special case of Fechner's principle of a tendency to stability, and Freud's pleasure principle is somewhat similar to one enunciated by Fechner.