The term “Newtonian particle” is introduced to designate the ultimate entities of which gross bodies are assumed to be constituted in Newton's mechanics. It is shown that the principle of mass-additivity can be validated more convincingly in that system if the mutual forces between pairs of Newtonian particles are noncentral than if these forces are central forces. On the basis of this conclusion the action and reaction pairs of the third law are identified as wrench-pairs rather than simple force-pairs.