Nothing will be said in this chapter about the development of the concept of the soul. The Homeric idea of the psyche or breath-soul as an insubstantial image of the body, giving it life and surviving it in a wretched, bloodless existence in Hades, is too familiar to need description here. Pythagoras was possibly the first Greek explicitly to treat the soul as something of moral importance, and Heraclitus first clearly indicated that some knowledge of the soul was relevant to knowledge of structure of the cosmos.
Yet the conception that the substance of the soul was related to aither, or to the substance of stars, seems from fifth-century B.C. poetical contexts to have existed for some time already as part of the complex body of popular beliefs, alongside the distinct Homeric concept of a breath-soul.
G.S. Kirk and J.E. Raven, The Presocratic Philosophers, Cambridge, At The University Press, 1971