in spite of starting upon the experimental and mathematical path, Newtonian science kept the idea that atoms are characterized by eternal properties or qualities, that is by essences. Substances are "solid, hard, massy, impenetrable, moveable particles." Their essence is precisely these unchange- able, fixed qualities of solidity, mass, motion, inertia. It thus appears that Newton retained apart of the qualita- tive equipment of the objects of Greek science, in spite of their irrelevance to both mathematics and experiment.