The Weber-Fechner Law, a fundamental concept in psychophysics explaining how the human senses perceive changes in stimuli. The core idea is that the ability to notice a difference (Just Noticeable Difference or JND) is relative to the initial intensity of the stimulus, rather than an absolute amount. Ernst Weber first observed this proportional relationship, and Gustav Fechner later formulated a logarithmic relationship between stimulus intensity and perceived sensation, known as Fechner's Law.

The texts also discuss sensory thresholds like the Absolute Threshold, provide everyday examples of these principles in action, and introduce Stevens's Power Law as an alternative model for scaling sensation. Finally, the sources highlight the practical applications of the Weber-Fechner Law in areas like marketing and design, while also acknowledging its limitations.