Inertia

Summary

The term “inertia” was first introduced by Kepler (Epitome of Copernican Astronomy, IV.3, 59; IV.2, 54, 55) to designate the opposition to motion that he considered natural to matter. One component of this opposition was a variant of Aristotelian doctrine about “violent motion” that Galileo, Gassendi, Beeckman, and Descartes would reject: persistence of motion requires some external agent, because bodies in motion tend toward rest. Although the Keplerian concept of inertia is sometimes erroneously identified with this first component alone, Kepler's view included a second component claim, a variant of which future physics would retain: bodies resist being put into motion, and their resistance varies with density (Kepler 1858 and 1896, 1:161; 6:174–75, 342; Jammer 1961, 57, 56, 55).

Descartes uses the term “inertia” much as Kepler did. In response to Debeaune's claim that resistance is due to a tardiveté naturelle, Descartes denies that bodies possess any such natural inertia or sluggishness. His own view emerging, he too acknowledges resistance, though correlating it with size rather than density, as his definition of matter in terms of extension alone demands. Diverging from Kepler, he pairs his recognition of resistance with the anti-Aristotelian claim that bodies in motion tend to stay in motion (AT II 466–67).

The term “inertia” eventually labeled a principle based upon Newton's first law of motion, which states, “Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by forces impressed” (Principia, 417). The concept of an inertial state is inseparable from such concepts as space and motion, and whereas Newton saw rest and uniform motion as distinct if humanly indistinguishable states, they were eventually classified as distinct only in connection with a chosen reference frame.