What was Galileo's argument against the old concept of Greek philosophers?

Galileo observed that on an inclined plane, the velocity of a body increases when it rolls down and decreases when it climbs up. He argued that on a plane inclined equally on both sides, a marble rolled from one side would attain the same height on the other side. He further argued that if the inclination of the plane on one side were gradually decreased, the marble would have to cover a larger distance to reach the same height and if the plane were made horizontal, the marble would continue to move forever in order to attain the same height. He challenged the age-old concept of Greek philosophers that the natural state of bodies is the state of rest by saying instead that, the natural tendency of bodies is to oppose a change in their state of motion.