The paper deals with the issue of formal ground and the external measure from the point of Newtonś Principia. By an analysis of the latter, by means of the Hegelian catogorial pairs reason – reasoned, the ground – grounded by, and formal ground - real ground, the author analyses both the Principia and Brian Ellis\' approach the concept of force as presented in his pathbreaking paper (1965). Finally it is proved that while mechanics is not a science which deals with the quality of forces, still it is capable of providing knowledge about the quantity of forces and about the quantity of their effects.