For centuries, the mental world of the mind and the physical world were treated as utterly distinct. While the movement of inanimate objects could be measured and ultimately predicted with the help of mathematics, the movement of organisms — their behavior — appeared to be shaped by different forces, under the control of the will.
About 200 years ago, the German physician Ernst Heinrich Weber made a seemingly innocuous observation which led to the birth of the discipline of Psychophysics
the science relating physical stimuli in the world and the sensations they evoke in the mind of a subject. Weber asked subjects to say which of two slightly different weights was heavier. From these experiments, he discovered that the probability that a subject will make the right choice only depends on the ratio between the weights.
For instance, if a subject is correct 75% of the time when comparing a weight of 1 Kg and a weight of 1.1 Kg, then she will also be correct 75% of the time when comparing two weights of 2 and 2.2 Kg
or, in general, any pair of weights where one is 10% heavier than the other. This simple but precise rule opened the door to the quantification of behavior in terms of mathematical ‘laws’.
Weber’s observations have since been generalized to all sensory modalities across many animal species, leading to what is now known as Weber’s Law. It is the oldest and most firmly established law in psychophysics. Psychophysical laws describe precise rules of perception and are important because they can be used to obtain mathematical explanations of behavior in terms of brain processes, just like the precise patterns of movement of the planets in the sky were useful to understand gravitation.