Anaximenes was a member of the Milesian school of philosophy founded by Thales, although they disagreed on one key point. Thales believed that water was the fundamental element from which everything came. Anaximenes believed it was air, based on the belief that people were held together by souls composed of air. Thin out the air and you have fire, condense it and you have clouds, condense it further and you have water and earth.
With Anaximander, Anaximenes proposed that the Earth was a flattened disk floating on air at the centre of the Universe. The Sun moved around the Earth (but not under it), and was also a flat body made of fire.