historically important psychological law quantifying the perception of change in a given stimulus. The law states that the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. It has been shown not to hold for extremes of stimulation.

The law was originally postulated to describe researches on weight lifting by the German physiologist Ernst Heinrich Weber (Weber, Ernst Heinrich) in 1834 and was later applied to the measurement of sensation by Weber's student Gustav Theodor Fechner (Fechner, Gustav Theodor), who went on to develop from the law the science of psychophysics. By stating a relationship between the spiritual and physical worlds, the law indicated to Fechner that there is really only one world, the spiritual. To others, the law meant the possibility of a scientific, quantitative psychology. The combined work of Weber and Fechner has been useful, especially in hearing and vision research, and has had an impact on attitude scaling and other testing and theoretical developments.