(born c. 540, Ephesus, in Anatoliadied c. 480 BC) Greek philosopher. Little is known of his life; the one book he apparently wrote is lost, and his views survive only in short fragments attributed to him. In his cosmology, fire forms the basic material principle of an orderly universe: he called the world order an ever-living fire kindling in measures and being extinguished in measures, and he extended fire's manifestations to include the ether in the upper atmosphere. The persistence of unity despite change is illustrated by his famous analogy of the river: Upon those who step into the same rivers, different and ever different waters flow down. Plato later took Heracleitus to mean that all things are in constant flux, regardless of how they appear to the senses.