What we now know as mass was until the time of Newton called “weight.”... A goldsmith believed that an ounce of gold was a quantity of gold.... But the ancients believed that a beam balance also measured “heaviness” which they recognized through their muscular senses.... Mass and its associated downward force were believed to be the same thing. Kepler formed a [distinct] concept of mass (“amount of matter” (copia materiae)), but called it “weight” as did everyone at that time.