Galilean Relativity

Galileo formulated his principle of relativity by stating that one cannot use any mechanical experiment to determine absolute constant uniform velocity. Now Maxwell's equations contain a velocity c but they do not specify with respect to what is this velocity to be measured!. We must conclude that either absolute velocities can be determined using experiments involving light, or else light must move at speed c in all reference frames.

But this is impossible to accept within Newtonian mechanics, for within this theory velocities simply add. If we then have a source of light moving at speed v, the light form it ought to travel at speed c + v in direct contradition to Maxwell's equations which predice that light travels with speed c, no matter how fast the speed of the source.