Pre-Socratic philosophy is Greek philosophy before Socrates (and includes schools contemporary with Socrates that were not influenced by him). In Classical antiquity, the Presocratic philosophers were called physiologoi (Greek: φυσιόλογοι; in English, physical or natural philosophers). Aristotle called them physikoi ("physicists", after physis, "nature") because they sought natural explanations for phenomena, as opposed to the earlier theologoi (theologians), whose philosophical basis was supernatural. Diogenes Laërtius divides the physiologoi into two groups, Ionian and Italiote, led by Anaximander and Pythagoras, respectively.
It may sometimes be difficult to determine the actual line of argument some Presocratics used in supporting their particular views. While most of them produced significant texts, none of the texts has survived in complete form. All that is available are quotations by later philosophers (often biased) and historians, and the occasional textual fragment.
The Presocratic philosophers rejected traditional mythological explanations of the phenomena they saw around them in favor of more rational explanations. These philosophers asked questions about "the essence of things":
- From where does everything come?
- From what is everything created?
- How do we explain the plurality of things found in nature?
- How might we describe nature mathematically?
Others concentrated on defining problems and paradoxes that became the basis for later mathematical, scientific and philosophic study.
Later philosophers rejected many of the answers the early Greek philosophers provided, but continued to place importance on their questions. Furthermore, the cosmologies proposed by them have been updated by later developments in science.
Western philosophy began in ancient Greece in the 6th century BCE. The Presocratics were mostly from the eastern or western fringes of the Greek world. Their efforts were directed to the investigation of the ultimate basis and essential nature of the external world. They sought the material principle (archê) of things, and the method of their origin and disappearance. As the first philosophers, they emphasized the rational unity of things, and rejected mythological explanations of the world. Only fragments of the original writings of the presocratics survive. The knowledge we have of them derives from accounts of later philosophical writers (especially Aristotle, Plutarch, Diogenes Laërtius, Stobaeus and Simplicius), and some early theologians (especially Clement of Alexandria and Hippolytus of Rome). The Presocratic thinkers present a discourse concerned with key areas of philosophical inquiry such as being and the cosmos, the primary stuff of the universe, the structure and function of the human soul, and the underlying principles governing perceptible phenomena, human knowledge and morality.
The first Presocratic philosophers were from Miletus on the western coast of Anatolia. Thales (624-546 BCE) is reputedly the father of Greek philosophy; he declared water to be the basis of all things. Next came Anaximander (610-546 BCE), the first writer on philosophy. He assumed as the first principle an undefined, unlimited substance without qualities, out of which the primary opposites, hot and cold, moist and dry, became differentiated. His younger contemporary, Anaximenes (585-525 BCE), took for his principle air, conceiving it as modified, by thickening and thinning, into fire, wind, clouds, water, and earth.
The practical side of philosophy was introduced by Pythagoras of Samos (582-496 BCE). Regarding the world as perfect harmony, dependent on number, he aimed at inducing humankind likewise to lead a harmonious life. His doctrine was adopted and extended by a large following of Pythagoreans who gathered at his school in south Italy in the town of Croton. His followers included Philolaus (470-380 BCE), Alcmaeon of Croton, and Archytas (428-347 BCE).
Heraclitus of Ephesus on the western coast of Anatolia in modern Turkey (535-475 BCE) posited that all things in nature are in a state of perpetual flux, connected by logical structure or pattern, which he termed Logos. To Heraclitus, fire, one of the four classical elements, motivates and substantiates this eternal pattern. From fire all things originate, and return to it again in a process of eternal cycles.
The Eleatic School, called after the town of Elea (modern name Velia in south Italy), emphasized the doctrine of the One. Xenophanes of Colophon (570-470 BCE) declared God to be the eternal unity, permeating the universe, and governing it by his thought.  Parmenides of Elea (510-440 BCE) affirmed the one unchanging existence to be alone true and capable of being conceived, and multitude and change to be an appearance without reality. This doctrine was defended by his younger countryman Zeno of Elea (490-430 BCE) in a polemic against the common opinion which sees in things multitude, becoming, and change. Zeno propounded a number of celebrated paradoxes, much debated by later philosophers, which try to show that supposing that there is any change or multiplicity leads to contradictions. Melissus of Samos (born c. 470 BCE) was another eminent member of this school.
Empedocles of Agrigentum (490-430 BCE) was from the ancient Greek city of Akragas (Ἀκράγας), Agrigentum in Latin, modern Agrigento, in Sicily. He appears to have been partly in agreement with the Eleatic School, partly in opposition to it. On the one hand, he maintained the unchangeable nature of substance; on the other, he supposes a plurality of such substances - i.e. four classical elements, earth, water, air, and fire. Of these the world is built up, by the agency of two ideal motive forces - love as the cause of union, strife as the cause of separation. Anaxagoras of Clazomenae (500-428 BCE) in Asia Minor also maintained the existence of an ordering principle as well as a material substance, and while regarding the latter as an infinite multitude of imperishable primary elements, he conceived divine reason or Mind (nous) as ordering them. He referred all generation and disappearance to mixture and resolution respectively. To him belongs the credit of first establishing philosophy at Athens.
The first explicitly materialistic system was formed by Leucippus (5th century BCE) and his pupil Democritus of Abdera (460-370 BCE) from Thrace. This was the doctrine of atoms - small primary bodies infinite in number, indivisible and imperishable, qualitatively similar, but distinguished by their shapes. Moving eternally through the infinite void, they collide and unite, thus generating objects which differ in accordance with the varieties, in number, size, shape, and arrangement, of the atoms which compose them.
The last of the Presocratic natural philosophers was Diogenes of Apollonia from Thrace (born c. 460 BCE). He was an eclectic philosopher who adopted many principles of the Milesian school, especially the single material principle, which he identified as air. He explained natural processes in reference to the rarefactions and condensations of this primary substance. He also adopted Anaxagoras' cosmic thought.
The Sophists held that all thought rests solely on the apprehensions of the senses and on subjective impression, and that therefore we have no other standards of action than convention for the individual. Specializing in rhetoric, the Sophists were more professional educators than philosophers. They flourished as a result of a special need at that time for Greek education. Prominent Sophists include Hippias (485-415 BCE) from Elis in the Peloponnesos, and Prodicus (465-390 BCE) from the island of Ceos.
Other early Greek philosophers
This list includes several men, particularly the Seven Sages, who appear to have been practical politicians and sources of epigrammatic wisdom, rather than speculative thinkers or philosophers in the modern sense.
- Solon (c. 594 BCE)
- Chilon of Sparta (c. 560 BCE)
- Thales (c. 585 BCE)
- Bias of Priene (c. 570 BCE)
- Cleobulus of Rhodes (c. 600 BCE)
- Pittacus of Mitylene (c. 600 BCE)
- Periander (625–585 BCE)
- Aristeas of Proconnesus (7th century BCE ?)
- Pherecydes of Syros (c. 540 BCE)
- Anacharsis (c. 590 BCE)
- The Pre-Socratic method of critical reasoning deployed in the examination of the natural world was applied by Socrates to an examination of the human individual and his social institutions.
- Karl Marx's doctoral thesis "The Difference Between the Democritean and Epicurean Philosophy of Nature" evaluates the thought of the Pre-Socratic philosopher Democritus, one of the founders of Atomic theory.
- Within the Marxist philosophical tradition the Pre-Socratics are recognized as the first Materialists.
- Nietzsche described the Pre-Socratics as The tyrants of the spirit, and says of Socrates that “the hitherto so wonderfully regular, although certainly too rapid, development of the philosophical science was destroyed in one night.”
- Karl Popper, one of the 20th Century's most influential philosophers of science, placed great importance on the critical tradition embodied in the development of Pre-Socratic thought, the analysis of which contributed to his own epistemological theories. His well-known essay on the subject "Back to the Pre-Socratics" can be found in the anthology of his essays, "Conjectures and Refutations - The Growth of Scientific Knowledge, 2nd Edition. Routledge Publishing. 2002.
- Daniel W. Graham (ed.), The Texts of Early Greek Philosophy: the Complete Fragments and Selected Testimonies of the Major Presocratics, 2 vols. (Cambridge; New York: Cambridge University Press, 2010).
- Presocratic Philosophy, Stanford Encyclopedia of Philosophy, 2007
- William Keith Chambers Guthrie, The Presocratic Tradition from Parmenides to Democritus, p. 13, ISBN 0-317-66577-4.
- John Freely, Before Galileo: The Birth of Modern Science in Medieval Europe (2012)
- Franco Orsucci, Changing Mind: Transitions in Natural and Artificial Environments, p. 14, ISBN 981-238-027-2.
- Eduard Feller, Outlines of the History of Greek Philosophy (1955). p. 323.
- Oskar Seyffert, (1894), Dictionary of Classical Antiquities, page 480
- Fredrich Nietzsche, Dawn, Aphorism 547
- Brisson, L. et al. Lire les Présocratiques. Presses universitaires de France, Paris, 2012.
- Burnet, John, Early Greek Philosophy, Meridian Books, New York, 1957
- Colli, Giorgio, The Greek Wisdom (La Sapienza greca, 3 vol. Milan 1977-1980)
- De Vogel, Cornelia J., Greek Philosophy, Volume I, Thales to Plato, E.J. Brill, Leiden, 1963
- (German) Diels, Hermann, Die Fragmente der Vorsokratiker, 6th ed., rev. by Walther Kranz (Berlin, 1952).
- (English) Freeman, Kathleen, Ancilla to the Pre-Socratic Philosophers: A Complete Translation of the Fragments in Diels, ISBN 978-1-60680-256-4 (Cambridge,  1970).
- Lloyd, G. E. R., Early Greek Science: Thales to Aristotle. New York: Norton, 1970.
- Kirk, G.S., Raven, J.E. & Schofield, M., The Presocratic Philosophers (Second Edition), Cambridge University Press, 1983
- Nahm, Milton C., Selections from Early Greek Philosophy, Appleton-Century-Crofts, Inc., 1962
- (French) Alain Sournia. Voyage en pays présocratique. Publibook, 2007.
- Giannis Stamatellos, Introduction to Presocratics: A Thematic Approach to Early Greek Philosophy with Key Readings, Wiley-Blackwell, 2012.
- Hussey, Edward, The Pre-Socratics, in series, Classical Life and Letters, G. Duckworth & Co., London, 1972. N.B.: A study of Pre-Socratic philosophy within the intellectual culture of Greek and other Antiquity. ISBN 0-7156-0824-X
- Luchte, James, Early Greek Thought: Before the Dawn, in series, Bloomsbury Studies in Ancient Philosophy, Bloomsbury Publishing, London, 2011. ISBN 978-0567353313
- Pre-Socratic philosophy at PhilPapers
- Pre-Socratic philosophy at the Indiana Philosophy Ontology Project
- Presocratic Philosophy entry by Patricia Curd in the Stanford Encyclopedia of Philosophy
- Pre-Socratic philosophy entry in the Internet Encyclopedia of Philosophy
- Presocratic Philosophy by Giannis Stamatellos
- (French) Philosophie sauvage by Alain Sournia