  The definitions inform the reader of how key technical terms, all of them designating quantities, are going to be used throughout the Principia. In the process Newton introduces terms that have remained a part of physics ever since, such as mass, inertia, and centripetal force. The emphasis in every one of the definitions is on how the designated quantity is to be measured, as illustrated by the opening definition: “Quantity of matter [or mass] is a measure of matter that arises from its density and volume jointly.” (Because a primary measure of density was then specific gravity, no circularity arises here.) Newton distinguishes among three ways of quantifying centripetal forces: the absolute quantity, which corresponds to what we would call the field strength of a central force field; the accelerative quantity, which “is the measure of this force that is proportional to the acceleration generated in a given time;” and the motive quantity, which is the measure of the force proportional to what we would call the change in linear momentum in a given time.