'Very well.' Isaac pointed to the circle in the middle of the diagram. 'Consider a spherical body – actually an aggregate of countless particles, each of which produces gravitational attraction according to an inverse square law.' Now he reached for the nearest handy object – an inkwell – and set it on the corner of the page, as far away from the 'spherical body' as it could go. 'What is felt by a satellite, here, on the outside, if the separate attractions of all of those particles are summed and fused into one aggregate force?'