1. Definitions

Our environment can be described by numerous physical quantities whose variations may or may not be perceptible to our senses. Light, for example, is simply an extremely rapid modification of a pair of electric and magnetic fields, while sound is a variation in the pressure of the air surrounding us. Our sense organs are biological devices that transform the excitations we perceive into variations in physical or chemical quantities that are internal to the organism and can thus be processed by the nervous system: they are sensors.