 DATE: When Frederick the Wise is directing Spalatin on how to  respond to the summons for Luther from the Cardinal, he talks about  letting the "inertia" of the situation take its course. Presumably  this conversation takes place on, or around, 1518 (and certainly  before the Diet of Worms in 1521), however, the term "inertia" was  first used by Johannes Kepler in works published from 1618 to 1621  nearly 100 years later.