Wind, simply put, is the movement of air in the atmosphere. Wind movement is caused by the fact that air will move from high pressure areas to low pressure areas. In other words, high pressure zones contain more densely packed molecules of various gases, which tend to flow to areas where the air is less dense. This concept was first explained by the Greek philosopher Anaximander (c. 610 B.C.E.–C. 546 B.C.E.), who explained that wind was a natural phenomenon and not caused by the gods or by trees waving their leaves, as some people thought.