He believed that air was the substance of all things. For example, when air is thinned out, he thought, it turns heated and becomes fire. When air is thickened, it condenses and becomes water and solid matter while when it condenses, it becomes earth. It is known now that these theories are wrong. But the idea that all things are made up of matter questioned the mind of yet another scientist, Leucippus. This led to one of the most important scientific discoveries introduced by the Greeks, the concept of atoms.