Full Idea
The attractions of the bodies must be reckoned by assigning proper forces to their individual particles and then taking the sums of those forces.
Gist of Idea
An attraction of a body is the sum of the forces of their particles
Source
Isaac Newton (Principia Mathematica [1687], 1.II.Schol)
Book Reference
Newton,Isaac: 'Philosophical Writings' [CUP 2004], p.86
A Reaction
This is using the parts of bodies to give fundamental explanations, rather than invoking substantial forms. The parts need not be atoms.