Transcript: It seems as if there are two fundamental ways of thinking about mass. One is inertial mass, the resistance an object presents to any change in its motion. Imagine trying to push a heavy object across a smooth surface. The other is gravitational mass, the force downward on an object caused by gravity. By the time of Einstein these two masses had been found to be utterly equivalent within the limits of measurement to one part in 1015 or less. To Einstein this could not be a coincidence, and so Einstein asserted that there was no way to distinguish between acceleration caused by gravity or by acceleration caused by any other force. This is the basis of his general theory of relativity.