From the known pre-Socratic philosophers Heraclitus stands out for both his originality and the range of his thought through the centuries. He was born in Ephesus around 540 B.C. and he came from the aristocratic family of Androcleidai. He participated in the political controversies of his city and expressed his contempt for the crowd, both in matters of political authority, and in those of religion. According to Diogenes Laertius Heraclitus he presented a copy of his book "Peri physeos" (on nature) to the temple of Ephesia Artemis. Obviously from there it was copied and spread quite early and it seems that it was already known to Parmenides and Democritus. It was written in Ionic dialect and in poetic form. Unfortunately, we know only fragments of it, and we cannot say with certainty that it is quoted word for word, since that meaning was unknown to ancient writers.

According to Heraclitean thought, truth -or rather truths are revealed to the philosopher only after obsessive pursuit and all of them are structured around the main idea of the unity of the world. As maintained by Heraclitus the world is unique, unborn and internal. These two elements that define its existence are the perpetual alteration and the constant conflict and unity of opposites. He calls this phenomenon enadiodromia or enadiotropia. The principle that governs the whole universy is the perpetual fire, that is the fire that regulates everything. To that issue the influence of the Ionian natural philosophers is obvious, but opposed to them Heraclitus conceives fire more as a rational principle, than as a prime cause of the world. The world exists through constant alteration of fire to air, liquid, earth and vice versa. The first procedure is called lower way and the second upper way. Heraclitus seemed to reject every concept about the immortality of the soul, because he considered that the soul was vapours of the liquid and warm elements and that after the death of the body they became water and earth. His complete cosmic theory was based on the conception of balanced and opposite pairs, the contrary connection of which brings harmony. Consequently, even the existence of gods and their immortality is not conceivable without men and mortality.

Heraclitus' work in the classical period already constituted part of the education of young men together with the Homeric epics. The width and the dynamics of his thinking and the unprecedented internal coherence of his theory appealed to great thinkers from different domains. For all the materialist philosophers his views about eternity and the uncreated of the world were decisive. On the other hand the designation of fire to an organizational principle preannounced Plato's idealism. Finally, the principles of unity and struggle of the opposites, found fundamental acceptance among most dialectics. From Plutarch to Spinoza and from Clement of Alexadria to J.L. Borges, Heraclitus is the thinker who exerted a unique charm, as his thought -exactly like the Heraclitean river- is never the same.