- A SKEPTIC's GUIDE - A SKEPTIC's GUIDE 
Prior to Newton, people who thought about such things observed that objects which had lots of inertia [i.e. were hard to get moving by pushing on them, even where a nearly frictionless horizontal motion was possible] were also invariably heavy [i.e. were pulled down toward the centre of the Earth with great force]. It was therefore understandable for them to have equated inertia with weight, the magnitude of the force of attraction to the Earth.3 Newton was among the first to suggest that inertia and weight were not necessarily the same thing, but that in fact the Earth's gravity just happened to pull down on objects with a force proportional to their inertial factor or "mass" (m) which was actually defined in terms of their resistance to horizontal acceleration by some force other than gravity.