 Physical space was the naïve, three-dimensional version of the space of Euclid’s Elements and of Cartesian coordinatised three-dimensional geometry, and this was how Newton had regarded it in his Principia Mathematica (1687). It was conceived of as a neutral arena, unaffected by matter or forces, but conditioning the movements of physical bodies under various kinds of forces. Chief among these was the force of gravity, which mathematicians in the Cartesian tradition regarded as a mysterious, even unacceptable, concept when it was introduced, but which by the start of the 19th century had been shown by Laplace to be capable of dealing well with all the known motions of the solar system. As a consequence, gravity had become a natural, primitive concept no longer in need of further explanation, and by 1800 it was reasonable of people who worked on the new theories of magnetism and electricity to consider them as forces and to model them, where appropriate, on Newtonian gravity.