Weber–Fechner law
Also known as: Weber's law, Fechner's law
Bigger baseline stimuli need proportionally bigger changes to be noticed.
What it means
The Weber–Fechner law captures the relationship between physical stimulus magnitude and perceived intensity. Weber's law states that the smallest detectable change in a stimulus is a constant proportion of its starting magnitude, so a heavier weight requires a larger added weight to feel different. Fechner built on this to propose that subjective sensation grows as the logarithm of stimulus intensity, compressing large physical ranges into a manageable perceptual scale. The framework launched psychophysics as a quantitative science of the mind and explains diminishing sensitivity across domains from brightness to price perception. Its logarithmic form is an approximation later refined by Stevens' power law, but the proportionality insight remains fundamental.
Examples
Adding one candle brightens a dark room dramatically, but the same candle is unnoticeable in a brightly lit hall.
Driving across town to save 10 pounds on a 25-pound kettle feels obvious; nobody makes the same trip to save 10 pounds on a 900-pound sofa.
Adding a paperback to an empty backpack is noticeable at once; slip the same book into a full hiking pack and your shoulders cannot tell the difference.
First described in Ernst Weber (1834); Gustav Fechner (1860).