Diognenes of Apollonia
Diogenes of Apollonia lived during the middle of the 5th century BCE and spent much time in Athens. He believed that air was the only substance, and it is air condensed or rarified, hot or cold, moist or dry, moving slower or fast which brings about all the variations in its forms. His reason for air being the only substance is because multiple substances wouldn't be able to interact and because all living beings breathe air, therefore it is the source of their soul and thought. He also studied the anatomy of blood vessels which also lead him to this conclusion about air. He also conjectured on how health, thought, and memory are related to the ability to take air throughout the body. He believed that there are countless worlds and infinite space. That there isn't anything that comes into being from nothing and there isn't anything that is destroyed into nothing.