Anaxagoras in the context of Ionian School (philosophy)

Pre-Socratic philosophy, also known as early Greek philosophy, is ancient Greek philosophy before Socrates. Pre-Socratic philosophers were mostly interested in cosmology, the beginning and the substance of the universe, but the inquiries of these early philosophers spanned the workings of the natural world as well as human society, ethics, and religion. They sought explanations based on natural law rather than the actions of gods. Their work and writing has been almost entirely lost. Knowledge of their views comes from testimonia, i.e. later authors' discussions of the work of pre-Socratics. Philosophy found fertile ground in the ancient Greek world because of the close ties with neighboring civilizations and the rise of autonomous civil entities, poleis.

Pre-Socratic philosophy began in the 6th century BC with the three Milesians: Thales, Anaximander, and Anaximenes. They all attributed the arche (a word that could take the meaning of "origin", "substance" or "principle") of the world to, respectively, water, apeiron (the unlimited), and air. Another three pre-Socratic philosophers came from nearby Ionian towns: Xenophanes, Heraclitus, and Pythagoras. Xenophanes is known for his critique of the anthropomorphism of gods. Heraclitus, who was notoriously difficult to understand, is known for his maxim on impermanence, ta panta rhei, and for attributing fire to be the arche of the world. Pythagoras created a cult-like following that advocated that the universe was made up of numbers. The Eleatic school (Parmenides, Zeno of Elea, and Melissus) followed in the 5th century BC. Parmenides claimed that only one thing exists and nothing can change. Zeno and Melissus mainly defended Parmenides' opinion. Anaxagoras and Empedocles offered a pluralistic account of how the universe was created. Leucippus and Democritus are known for their atomism, and their views that only void and matter exist. The Sophists advanced philosophical relativism. The Pre-Socratics have had significant impact on several concepts of Western philosophy, such as naturalism and rationalism, and paved the way for scientific methodology.

The Ionian school of pre-Socratic philosophy refers to Ancient Greek philosophers, or a school of thought, in Ionia in the 6th century BC, the first in the Western tradition.

The Ionian school included such thinkers as Thales, Anaximander, Anaximenes, Heraclitus, Anaxagoras, and Archelaus. This classification can be traced to the doxographer Sotion. The doxographer Diogenes Laërtius divides pre-Socratic philosophy into the Ionian and Italian school. The collective affinity of the Ionians was first acknowledged by Aristotle who called them physiologoi (φυσιολόγοι), or natural philosophers. They are sometimes referred to as cosmologists, since they studied stars and maths, gave cosmogonies and were largely physicalists who tried to explain the nature of matter.

Aristarchus of Samos (/ˌærəˈstɑːrkəs/; Ancient Greek: Ἀρίσταρχος ὁ Σάμιος, Aristarkhos ho Samios; c. 310 – c. 230 BC) was an ancient Greek astronomer and mathematician who presented the first known heliocentric model that placed the Sun at the center of the universe, with the Earth revolving around the Sun once a year and rotating about its axis once a day. He also supported the theory of Anaxagoras that the Sun was just another star.

He likely moved to Alexandria, and he was a student of Strato of Lampsacus, who later became the head of the Peripatetic school in Greece. According to Ptolemy, Aristarchus observed the summer solstice of 280 BC. Vitruvius writes that Aristarchus built two different sundials: one a flat disc; and one hemispherical. Aristarchus estimated the sizes of the Sun and Moon as compared to Earth, and the distances from the Earth to the Sun and to the Moon. His estimate that the Sun was 7 times larger than Earth (actually 109 times) brought about the further insight that the Sun's greater size made it the most natural central point of the universe, as opposed to Earth.

Satyrus (Ancient Greek: Σάτυρος) of Callatis was a distinguished Peripatetic philosopher and historian, whose biographies of famous people are frequently referred to by Diogenes Laërtius and Athenaeus. He came from Callatis Pontica, as was learned from a Herculaneum papyrus. He lived earlier than the reign of Ptolemy VI Philometor (181–146 BC) when his Lives were epitomized by Heraclides Lembus, probably during the 3rd century BC. Athenaeus frequently refers to him as a Peripatetic, but his connection to the Peripatetic school is otherwise unknown. His biographies dealt with many eminent people including kings (Dionysius the Younger, Philip), statesmen (Alcibiades), orators (Demosthenes), poets (Aeschylus, Sophocles, Euripides), and philosophers (Bias of Priene, Chilon of Sparta, Pythagoras, Empedocles, Zeno of Elea, Anaxagoras, Socrates, Diogenes, Anaxarchus, Stilpo). He also wrote on the population of Alexandria, and a work On Characters (Περὶ χαρακτήρων). Fragments of his biography of the Athenian dramatist Euripides were found at the end of a papyrus scroll discovered at Oxyrhynchus in the early twentieth century.

Archelaus (/ɑːrkɪˈleɪəs/; Greek: Ἀρχέλαος; fl. 5th century BC) was an Ancient Greek philosopher, a pupil of Anaxagoras, and may have been a teacher of Socrates. He asserted that the principle of motion was the separation of hot from cold, from which he endeavoured to explain the formation of the Earth and the creation of animals and humans.