The junior member of the Miletian school, and probably a pupil of Anaximander. His astronomy was relatively unsophisticated, but he is remembered for the doctrine that one primary substance, aer, produces all others either by being rarefied into fire or condensed into wind, cloud, water, earth, and stone. This is the first physical account in the western tradition of different substances as modifications of one primary stuff. The phenomenon that impressed Anaximenes was that breath can blow warm (when it is rarefied, i.e. the mouth is open) or cold (when it is compressed, or hissed out). See also atomism, materia prima.