Life evolved from the primeval world of physics. Sensory systems inform animals of the natural environment, enabling them to conduct responsively. The discovery of weak, DC bioelectric fields in the vicinity of aquatic organisms and the role they play in guiding sharks and rays to their prey have led to the recognition of fundamental, hitherto less well known, physical aspects of sensory biology. The inferred cybernetic algorithm of electric-field orientation in sharks and rays is highly effective and extremely robust. In orienting to the weak DC electric fields of ocean currents and to the earth's magnetic field, sharks and rays unwittingly practise the motional-electric principles that Einstein had in mind when he introduced the special theory of relativity. At the sense-organ, receptor-membrane, and ion-channel levels, the elasmobranch ampullae of Lorenzini operate on the basis of graded positive feedback driven by negative conductance, supposedly employing voltage-sensitive ion channels as the active, excitable elements. The electric sense of sharks and rays presents an exquisite implementation of the very biophysical principles that also govern the graded, much richer than all-or-none, integrative brain processes of animal and man.