Anaximene was a pre-Socratic philosopher from ancient Greece, known for his contributions to early cosmology and metaphysics. He lived in the 6th century BCE and was a student of Anaximander. Anaximene proposed that air, or "aer," is the fundamental substance of the universe, from which everything else originates. He believed that by processes of rarefaction and condensation, air transforms into different elements like fire, water, and earth. His ideas were significant in the development of natural philosophy, as they shifted the focus from mythological explanations of the world to more rational, observable phenomena. Anaximene's work laid the groundwork for later thinkers, influencing figures such as Heraclitus and Aristotle.