How did Galileo idealize his inclines to conclude that an object in motion stays in motion until it is acted on by some force?

Aristotle thought that when you drop 2 things with different masses that the heavier one would hit the ground first. Galileo proved him wrong by rolling two balls of different masses down an incline plane and timing them using a water clock. He found that the mass of an object does not affect how quickly it accelerates due to gravity.