Gravity causes all matter to be attracted to other matter, from the subatomic to the cosmic levels. The earliest people could observe gravity at work, noticing objects falling to earth, but they did not begin to theorize systematically about the reasons behind such motion until the era of Classical Greece. The discovery of how gravity works progressed in several stages, beginning with Democritus and proceeding through the work of al-Hasan ibn al-Haytham, Galileo Galilei and Sir Isaac Newton.