Empedocles (c.494 – c.434 BC) was a Greek pre-Socratic philosopher and a native citizen of Akragas, a Greek city in Sicily (now Agrigento). Empedocles' philosophy is best known for originating the cosmogonic theory of the 4 classical elements. Empedocles developed a distinctive doctrine of reincarnation & was vegetarian. Empedocles also proposed forces he called Love and Strife which would mix and separate the elements, respectively.

Anaxagoras (c. 500- c. 428 BC) was a Pre-Socratic Greek philosopher. Anaxagoras described the world as a mixture (Apeiron) of primary imperishable ingredients, where material variation was never caused by an absolute presence of a particular ingredient, but by its relative preponderance over other ingredients; Anaxagoras introduced the concept of Nous (Cosmic Mind) as an ordering force, which moved and separated out the original mixture.

Parmenides of Elea (born c. 515 BC) was a pre-Socratic Greek philosopher from Elea in Magna Graecia (in Southern Italy). Parmenides has been considered the founder of ontology or metaphysics and has influenced the whole history of Western philosophy. He was the founder of the Eleatic school of philosophy. The single known work by Parmenides is a poem, often referred to as On Nature:

Xenophanes of Colophon (c. 570 – c. 478 BC) was a Greek philosopher, theologian, poet, and critic of religious polytheism. Xenophanes rejected the traditional views of the Olympian gods, and claimed a supreme non-anthropomorphic God who controls the cosmos by thought. Xenophanes wrote in verse, and while some of the surviving fragments deal with typical poetic topics, he also addressed theological and philosophical questions.

Heraclitus of Ephesus (c.535 – c.475 BC) was an Ancient Greek, pre-Socratic, Ionian philosopher and a native of the city of Ephesus, which was then part of the Persian Empire. His paradoxical philosophy and appreciation for wordplay and cryptic utterances has earned him the epithet "The Obscure" since antiquity. Heraclitus wrote a single work, only fragments of which have survived, increasing the obscurity associated with him.

Heraclitus (c.535 – c.475 BC) was born at Ephesus, apparently from a noble family connected with religious rites, but early retired from their social position and devoted himself to study and the development of his philosophical ideas. From his book only fragmentary quotations have survived as citations from later authors. This article contains all the fragments which can authoritatively be ascribed to Heraclitus.