The last of the Milesian school was Anaximenes. He, as Thales and Anaximander before him, tried to identify the single substance out of which everything is made. While Thales and Anaximander thought it was water and apeiron (indefinite) accordingly, Anaximenes opted for air, pointing out that just as air gives life to the human body, so a universal kind of air gives life to the cosmos.
His dates are very uncertain. Since we know that he was Anaximander’s student, and that he certainly flourished before 494 BC, when Miletus had been destroyed by the Persians, we can say he lived around 585-528 BC.
Anaximenes based his theory on such observable processes as rarefaction and condensation. He, actually, was the first thinker on record to support his ideas with evidence. Blowing with pursed lips produced cold air; with relaxed lips, warm air. Then, when something condenses, it cools; when it expands, it heats up. In same manner, when air is condensed, it becomes visible, transforming into mist, rain, earth, and ultimately rock. Vice versa, when water evaporates into air and ignites, it forms fire.
As Bertrand Russell points out (The History of Western Philosophy): “Anaximenes’ theory has the merit of making all the differences between different substances quantative, depending entirely upon the degree of condensation.”
On the whole, Anaximenes’ theory resembled Thales’. But he also thought of this Air as boundless and as containing an infinite number of worlds, in this respect following his teacher, Anaximander.
Anaximenes also has his own theory of genesis of Earth. The air condensed to create the flat disk of the Earth, which he said was table-like and floated on air like a leaf. He viewed celestial bodies, just like his predecessors, as fiery balls: Earth let out an exhalation of air that rarified, ignited and became the stars. Sun was not rarified air, but rather consisted of earth, like Moon. Anaximenes attributed Sun’s flame to the speed of motion. Just like Earth, Sun and Moon are flat disks, floating on streams of air. When Sun sets down it does not go under the Earth, but is overshadowed by the higher areas of Earth, as it circles around and distances away.
As John Burnet says (Early Greek Philosophy): “The heavenly bodies also afloat on the air. Their paths are not oblique, but the earth is tilted up, so that most of them are hidden when they get behind the higher side of it.It is unfortunate that Anaximenes did not know the spherical shape of the earth; for this line of thought might have led him to discover the inclination of its axis.”
Anaximenes had tried, like Anaximander, to explain natural phenomena. For example, he thought lightning is caused by the wind, which violently separates clouds, creating a bright flash. He also explained earthquakes as a result either of lack of moisture, which causes the earth to break apart because of how parched it is, or of overabundance of water.
The Milesian School came to an end with the fall of Miletus in 494 BC, as was written above. It is important for what it tried to achieve, not for what it really achieved. The speculations of Thales, Anaximander and Anaximenes had a profound influence on all later philosophers.
1) Bertrand Russell. The History of Western Philosophy.
2) John Burnet. Early Greek Philosophy.
3) Dorling Kindersley Limited (2011). The Philosophy Book.
4) Kathleen Freeman. Ancilla to the Pre-Socratic Philosophers.