or Weber-Fechner law In psychophysics, a historically important law quantifying the perception of change in a given stimulus. Originated by the German physiologist Ernst Heinrich Weber (1795-1878) in 1834 and elaborated by his student Gustav Theodor Fechner, the law states that the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. It was later shown not to hold for extremes of stimulation