Chen-Ning Yang considered the AB effect to be only direct experimental proof of the gauge principle. The philosophical import is that the magnetic four-potential ( ϕ ϕ , A ) overdescribes the physics, in that all observable phenomena are the same after we perform a gauge transformation. The Maxwell fields underdescribe the physics, as it does not predict the AB effect. And just as the gauge principle predicts, the quantities that are invariant under gauge transforms are exactly the physically observable phenomena.