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Gustav Theodor Fechner was one of the outstanding German scientists and thinkers. He is well known as the eminent founder of a new science "psychophysics" - the quantitative study of the relations between physical stimuli and sensations. But it seems that first ideas and first solutions of this new science are not the result of hard experimental work but rather of metaphysical speculations. So we found for the first time the important "Fundamentalformel" in the philosophical book "Zend-Avesta", written by Fechner already in 1851. Therefore this formula may not be the result of his later experimental efforts, put down in writing in the important "Elemente der Psychophysik" (1860). In the present paper it was intended to retrace the so-called "indefinite train of thoughts" (Fechner) that lead him to his strictly mathematical formula.