Anaximenes was a pre-Socratic philosopher from Miletus, active in the 6th century BCE. He is best known for proposing that air is the fundamental substance of the universe, from which everything else derives. Anaximenes believed that changes in the density of air could explain the formation of different materials and phenomena. He was a student of Anaximander and contributed to the early development of natural philosophy. Anaximenes' ideas laid the groundwork for later thinkers, influencing the way subsequent philosophers, such as Heraclitus and Democritus, approached the nature of reality and existence.