Diogenes of Apollonia (5th century BC)

Diogenes was the last of the early Greek physicists. He claimed that interactions between things would be impossible unless all were forms of one basic substance. Adapting ideas of Anaximenes and Anaxagoras, he identified the basic substance as air, which in its optimal form possesses intelligence and thereby controls the universe at large and animal life in particular. Diogenes worked out a detailed psychology and physiology, explaining sense perception as an exercise of intelligence due to interaction between air in the region of the brain and atmospheric air. This theory was mocked by Aristophanes, but influenced various Hippocratic writings.