In the ancient Greek philosophy, arche (ἀρχή) is the beginning or the first principle of the world. Ancient Greek philosophy focused on the role of Reason and Inquiry. The idea of an arche was first philosophized by Thales of Miletus, who claimed that the first principle of all things is water. Thales of Miletus According to Bertrand Russell, "Philosophy begins with Thales Miletus (mī lē' təs ( Ancient Greek: Μίλητος literally Transliterated Milētos, Latin Miletus) was an Ancient His theory was supported by the observation of moisture throughout the world and coincided with his theory that the earth floated on water.

Thales' theory was refuted by his successor and estimated pupil, Anaximander. Anaximander ( Ancient Greek:) (c 610 BC–c 546 BC was a Pre-Socratic Greek philosopher who lived in Miletus Anaximander noted that water could not be the arche because it could not give rise to its opposite, fire. Anaximander claimed that none of the elements (earth, fire, air, water) could be arche for the same reason. Instead, he proposed the existence of the apeiron, an indefinite substance from which all things are born and to which all things will return.

Anaximenes, Anaximander's pupil, advanced yet another theory. Anaximenes (Άναξιμένης of Miletus (c 585 BC-c 525 BC was a Greek Pre-Socratic Philosopher from the latter half of the He returns to the elemental theory, but this time posits air, rather than water, as the arche. Anaximenes suggests that all is made from air through either rarefication or condensation (thinning or thickening). Rarefied, air becomes fire; condensed, it becomes first wind, then cloud, water, earth, and stone in order. The Arche is technically what underlies all of reality/appearances.