In his Principia , Newton explained his system of how gravity , previously thought of as an occult force ( that is , an unexplained force ) , directed the movements of celestial bodies .
In his Principia , Newton kept our solar system in its working order .
In his Principia , Newton explained his system of how gravity , previously thought of as an occult force ( that is , an unexplained force ) , directed the movements of celestial bodies , and kept our solar system in its working order .