Anaximander, Anaximenes and the Milesian Debate

Thales' fellow citizens Anaximander and Anaximenes turned his question into a debate, the first sign of a philosophical tradition. Anaximander, active around 560 BCE, argued that no ordinary element could be the source of everything, because water, fire and the rest are opposites that would destroy each other. Instead he proposed the apeiron, the boundless or indefinite, from which opposites separate out and to which they return.

He also pictured the Earth floating unsupported at the center of the cosmos, staying put because it had no reason to move one way rather than another. Anaximenes then proposed air as the source, explaining change by a process: air condenses into wind, cloud, water and earth, and thins into fire. This was significant because it explained how one substance becomes many. Together the Milesians show philosophers building on, and correcting, each other.