Cer- tain properties regarded by Newton as inherent in substances and essential to them, in independence of connectivity, were indeed speedily seen to be relations. This conversion hap- pened first as to hardness and impenetrability, which were seen to be reducible to mass. Vis inertiae was ameasure of mass. By careful thinkers "force" was treated as ameasure of acceleration and so aname for arelation, not as an inherent property of an isolated thing by virtue of which one thing could compel another to change.