Fechner's Law is a principle in psychology that describes the relationship between the intensity of a stimulus and the perception of that stimulus. It states that the perceived change in a stimulus is proportional to the logarithm of the actual change in intensity. This means that as a stimulus increases, the perceived difference becomes less noticeable. The law was formulated by Gustav Fechner, a German philosopher and psychologist, in the 19th century. It is foundational in the field of psychophysics, which studies how physical stimuli are translated into sensory experiences. Fechner's Law helps explain why we may not notice small changes in loudness or brightness as easily as larger changes.