In classical mechanics, the acceleration of objects has to be prescribed before the resulting motions may be computed. In fact, the mathematical formula that specifies how objects respond to forces, the ‘equation of motion’, can be read in this way: The acceleration experienced by any object is equal to the net force exerted on that object, divided by its mass.

The cause of the acceleration is not part of the system proper. It must be specified separately, put in from the outside, so to speak. When the science of mechanics was developed, this requirement led to a wild variety of hypotheses about the properties and causes of accelerations, to the tune of fierce and often acrimonious debate. In fact, the whole concept of ‘force’ had a bad reputation. It was much too vague, and carried the odium of magic and arbitrariness.

Of course every blacksmith, carpenter and bricklayer of that time knew that the forces among material objects were somehow related to their structure. The opinions of craftspeople and engineers, however, were not held in high esteem, except by people like Stevin and Huygens, who were very skilled in engineering and practical work.

Nor did it help that Descartes made a fine mess of it with his vortices of hypothetical ghost particles. Armchair philosophers could, and usually did, invent a new particle for every phenomenon in the world, resulting in a Shakespearean ‘sound and fury, signifying nothing’.

So when Newton declared that ‘gravity’ was the ‘universal’ source of accelerations, he did something quite daring. To his and our good fortune, he also cast the expression for his ‘universal force’ in an astonishingly simple mathematical form: the acceleration is inversely proportional to the square of the distance and independent of the accelerated object's mass. The latter requirement was absolutely necessary because of Stevin's experiment in 1585, about a century before Newton's work.

Scientists who are sensitive to such things find this simplicity a source of great beauty. It seems instantly convincing because it is the diametrical opposite of all the vague Cartesian stuff about ethereal particles and vortices, which contains no more physics than the ‘epicycles’ of antiquity.