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mente der Psychophysik) by appealing to the principle of the conservation of physical energy, and he is the first to have applied this principle to the relation of soul and body. He thinks that the brain and nervous system, like all matter, must come under this principle, and that the ordinary assumption of a real interaction of spirit and matter cannot therefore be correct, because in that case physical energy would begin and cease. The relation is rather one of identity, and the distinction depends on the viewpoint of the observer. Just as the observer standing on the external surface of a sphere sees nothing but convexity, and one standing on the internal surface sees only concavity, so the materialist sees nothing but matter and the idealist only spirit—and both are right, each from his own viewpoint.—The resulting problem then is, what quantitative relations do the psychical phenomena sustain to their corresponding material phenomena? Fechner thinks that this relation cannot be one of direct proportion, but that it must be logarithmic, i.e. the psychical changes correspond quantitatively to the relation of the increase of its corresponding material process and the processes already present. Fechner thus assumes that the relation between the external stimulus and the brain process to which it gives rise is directly proportional, because both are material events, but the relation between the psychical process of sensation and the brain process on the other hand must be logarithmic. He regarded Weber's Law (so called in honor of his precursor, the physiologist E. H. Weber), which he assumed and verified experimentally, as an expression of the relation of spirit and matter in general. Upon the basis of experiments of his own as well as of others, on the relation of sensation and stimulus, he found that his law applied