A number of early Greek philosophers active before and during the time of Socrates are collectively known as the pre-Socratics. Their inquiries spanned the workings of the natural world as well as human society, ethics, and religion, seeking explanations based on natural principles rather than the actions of supernatural gods. They introduced to the West the notion of the world as a kosmos, an ordered arrangement that could be understood via rational inquiry.
In Classical antiquity, the pre-Socratic philosophers were called physiologoi (Greek: ?; in English, physical or natural philosophers).Aristotle was the first to make a clear distinction between these physiologoi or physikoi ("physicists", after physis, "nature") who sought natural explanations for phenomena, and the earlier theologoi (theologians), or mythologoi (story tellers and bards) who attributed these phenomena to various gods.Diogenes Laërtius divides the physiologoi into two groups: Ionian, led by Anaximander, and the Italiote, led by Pythagoras.
Hermann Diels popularized the term "pre-Socratic" in Die Fragmente der Vorsokratiker (The Fragments of the Pre-Socratics) in 1903. However, the term "pre-Sokratic" [sic] was in use as early as George Grote's Plato and the Other Companions of Sokrates in 1865. Edouard Zeller was also important in dividing thought before and after Socrates. Major analyses of pre-Socratic thought have been made by Gregory Vlastos, Jonathan Barnes, and Friedrich Nietzsche in his Philosophy in the Tragic Age of the Greeks.
It may sometimes be difficult to determine the actual line of argument some pre-Socratics 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 pre-Socratic 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":
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.
Coming from the eastern or western fringes of the Greek world, the pre-Socratics were the forerunners of what became Western philosophy and natural philosophy, which later developed into the natural sciences (such as physics, chemistry, geology, and astronomy). 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 supernatural explanations, instead seeking natural principles at work in the world and human society. The pre-Socratics saw the world as a kosmos, an ordered arrangement that could be understood via rational inquiry. pre-Socratic thinkers present a discourse concerned with key areas of philosophical inquiry such as being, 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.
Only fragments of the original writings of the pre-Socratics survive (many entitled Peri Physeos, or On Nature, a title probably attributed later by other authors). The knowledge we have of them derives from accounts - known as doxography - of later philosophical writers (especially Aristotle, Plutarch, Diogenes Laërtius, Stobaeus and Simplicius), and some early theologians (especially Clement of Alexandria and Hippolytus of Rome).
However, the translation of Peri Physeos as On Nature may be misleading: the "on" normally gives the idea of an "erudite dissertation", while "peri" may refer in fact to a "circular approach"; and the traditional meanings of "nature" for us (as opposition to culture, to supernatural, or as essence, substance, opposed to accident, etc.) may be in contrast with the meaning of "physeos" or "physis" for the Greeks (referring to an "originary source", or "process of emergence and development").
The first pre-Socratic philosophers were from Miletus on the western coast of Anatolia. Thales (624-546 BC) is reputedly the father of Greek philosophy; he declared water to be the basis of all things. Next came Anaximander (610-546 BC), the first writer on philosophy. He assumed as the first principle an undefined, unlimited substance without qualities (apeiron), out of which the primary opposites, hot and cold, moist and dry, became differentiated. His younger contemporary, Anaximenes (585-525 BC), 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 BC). 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 BC), Alcmaeon of Croton, and Archytas (428-347 BC).
Heraclitus of Ephesus on the western coast of Anatolia in modern Turkey (535-475 BC) 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 southern Italy), emphasized the doctrine of the One. Xenophanes of Colophon (570-470 BC) declared God to be the eternal unity, permeating the universe, and governing it by his thought.Parmenides of Elea (510-440 BC) 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 BC) 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 BC) was another eminent member of this school.
Empedocles of Agrigentum (490-430 BC) was from the ancient Greek city of Akragas (?), 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 BC) 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 BC) and his pupil Democritus of Abdera (460-370 BC) 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 pre-Socratic natural philosophers was Diogenes of Apollonia from Thrace (born c. 460 BC). 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 Protagoras (490-420 BC) from Abdera in Thrace, Gorgias (487-376 BC) from Leontini in Sicily, Hippias (485-415 BC) from Elis in the Peloponnesos, Prodicus (465-390 BC) from the island of Ceos, and Thrasymachus (459-400 BC) from Chalcedon on the Bosphorus.
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.