Several models have been proposed in the field of psychophysics to quantify relationships between any stimulus (e.g., touch, sound, light, and smell) and the perceived response by individuals. One such model is referred to as the Weber-Fechner Law. The Weber −Fechner Law, however, is not one law, but two separate laws: Weber's Law and Fechner's Law. Moreover, not all human senses respond to stimuli according to Fechner's law (in fact many do not). Weber's Law and special cases such as Fechner's Law are each based on the “just noticeable difference threshold” concept.