Minimal coupling and Feynman's proof

The non quantum relativistic version of the proof of Feynman for the Maxwel l equations is discussed in a framework with a minimum number of hypotheses required. From the present point of view it is clear that the classical eq uations of motion corresponding to the gauge field interactions can be dedu ced from the minimal coupling rule, and we claim here resides the essence o f the proof of Feynman.