Treating Earth as a system in which life plays an important role in controlling the environment is not new. In fact, speculation on how the physical and living elements of the Earth interact goes back to antiquity, and earlier, with deities capriciously intervening in the affairs of the human population. The notion of a single integrated system mediated by gods was the stuff of religion and legend. The Greek cosmologists dismissed arbitrary gods as controllers of the universe. Instead, they considered that the order of nature and the cosmos follows eternal cycles. Democritus (c. 460–370 BCE) proposed an atomist theory of matter that invoked an early form of the conservation of energy: atoms are in eternal motion. The Roman poet and philosopher Lucretius (96–55 BCE) drew on the atomic theory of Democritus to account for a variety of terrestrial phenomena, such as earthquakes and lightning, according to physical principles. Some Greek philosophers, including Thales (624–546 BCE), Anaximenes (585–528 BCE) and Heraclitus (535–475 BCE), taught a philosophical doctrine that there is a form of life in all matter. The rationalist philosophical styles of the ancient world were lost until the world of ideas was revived in Europe in the fifteenth century by the rise of humanism in Italy.