Why was Aristotle's law of motion discredited?

"Galileo was the first to really discredit Aristotle's laws. He did this by postulating that heavier objects would hit the ground at the same time as lighter objects, when dropped from the same height. He proved this with a famous demonstration where he dropped two balls of different weights from the leaning tower of Pisa. The balls hit the ground at the same time. Galileo also came up with the idea of inertia. Rather than take Aristotle's view that and object must have a force acting on it to keep moving, Galileo said that an object would move continuously in a straight line as long as no outside forces acted on it."