  Maxwell's mature presentation of his equations emphasized the unity of
electromagnetism and mechanics, subsuming both as "dynamical systems". That
intuition of unity has proved both fruitful, as a source of pregnant concepts,
and broadly inspiring. A deep aspect of Maxwell's work is its use of redundant
potentials, and the associated requirement of gauge symmetry. Those concepts
have become central to our present understanding of fundamental physics, but
they can appear to be rather formal and esoteric. Here I discuss two things:
The physical significance of gauge invariance, in broad terms; and some
tantalizing prospects for further unification, building on that concept, that
are visible on the horizon today. If those prospects are realized, Maxwell's
vision of the unity of field and substance will be brought to a new level.
