History of the philosophy of field theory  Field theory had its origins in the 18th century in a mathematical formulation of Newtonian mechanics, but it was seen as deficient as it implied action at a distance. In 1852, Michael Faraday treated the magnetic field as a physical object, reasoning about lines of force. James Clerk Maxwell used Faraday's conceptualisation to help formulate his unification of electricity and magnetism in his electromagnetic theory. With Albert Einstein's special relativity and the Michelson-Morley experiment, it became clear that electromagnetic waves did not travel as vibrations in a physical aether; and there was in Einstein's physics no difference between the effects of a field and action at a distance. In quantum field theory, fields become the fundamental objects of study, and particles are excitations of these fields. Field theory, the study of dynamical fields in physics, was originally a mathematical formulation of Newtonian mechanics. The success of Newtonian physics since the publication of Isaac Newton's "Principia" in 1687 provided a framework with which to investigate the motion and forces associated with electricity and magnetism. Charles-Augustin de Coulomb showed in 1785 that the repulsive force between two electrically charged spheres obeys the same (up to a sign) force law as Newton's law of universal gravitation: the force between two bodies is directed along the line separating the bodies and its magnitude is proportional to the product of their charges (for gravitation, their masses) divided by the square of their distance apart. André-Marie Ampère showed in 1823 that the force between infinitesimal lengths of current-carrying wires similarly obeys an inverse-square law such that the force is directed along the line of separation between the wire elements. Despite the success of these theories in making accurate numerical predictions of a wide range of phenomena, they were generally seen as deficient as natural philosophies of mechanics, since they were all essentially action-at-a-distance mechanisms. In the context of the development of field theory, the fact that a function could be written to give the force per unit mass, charge, or current for each point in space was just a mathematical construct. It was seen as untenable on metaphysical grounds that a force be exerted across empty space, and hence these force laws were assumed to be merely descriptive and not explanatory. To explain the apparent action-at-a-distance and provide mechanics with a grounding in metaphysics, in 1786 Immanuel Kant proposed a more general definition of matter:  "Matter is whatever is movable and fills a space.