"Traditionally, [physics] had concerned itself with searching out quantitative relationships between measurable phenomena... To go beyond this, to explain observable facts in terms of unobservable but alledgedly "real" entities such as atoms, was to go beyond what many physicists regarded as the limits of their discipline. What was happening in the second half of the 19th century, was the birth of the subject we now call theoretical physics... a puzzling innovation. ...As well as having a hand in kinetic theory, ...In 1864, ...Maxwell's theory introduced a new idea, the electromagnetic field. Over atoms and electromagnetic fields, the same question arose: real or imaginary?"