One result in particular of these investigations in the physiology of the senses became the starting point of a strong new movement. The course of biology in the second quarter of the nineteenth century was toward methodical and exact study of empirical facts, and away from speculation in the philosophy of nature. But for some time this exact study and this speculation were often to be found combined in the same men. Fechner was one of these. On the one hand he was a speculative philosopher, a follower of Schelling’s philosophy of nature, a disciple of Herbart in his attempt at applying mathematics to psychology. So we find him speculating as to what might be the exact relations between body and soul, seeking for a mathematical formulation of the dependence of the corresponding mental and nervous processes. One October morning in 1850, while lying in bed, he conceived a formula which seemed to him plausible. In spite of this speculative tendency he was a physicist of scientific exactness, accustomed to demand a support of facts for such plausible formulas, ready to attack problems not only with his mind, but also with his hands. In following up his speculations he came across some of the results of the work of Weber. By the use of more exact methods and by long-continued series of experiments he carried Weber’s investigations farther, at the same time utilizing the observations of others to which no one had before paid any attention. He succeeded in formulating the first mathematical law of mental life, Weber’s law as he called it, according to which an increase of the external stimulus in geometrical progression corresponds to the increase of the mental process in arithmetical progression. (We shall discuss this law in § 4.) He classed together all of his speculations, investigations, formulations, and conclusions as a new branch of knowledge, Psychophysics, “the scientific doctrine of the relations obtaining between body and mind.”