If one looks at the different problems of the integral calculus which arise naturally
when one wishes to go deep into the different parts of physics, it is impossible
not to be struck by the analogies existing. Whether it be electrostatics or
electrodynamics, the propagation of heat, optics, elasticity, or hydrodynamics,
we are led always to differential equations of the same family. (Henri Poincare,
circa 1890)