Western philosophical tradition began in ancient Greece in the 6th century BCE. The first philosophers are called Presocratics which designates that they came before Socrates. The Presocratics were from either the eastern or western regions of the Greek world. Athens -- home of Socrates, Plato and Aristotle -- is in the central Greek region and was late in joining the philosophical game. The Presocratic's most distinguishing feature is emphasis on questions of physics; indeed, Aristotle refers to them as Investigators of Nature. Their scientific interests included mathematics, astronomy, and biology. As the first philosophers, though, they emphasized the rational unity of things, and rejected mythological explanations of the world. Only fragments of the original writings of the presocratics survive, in some cases merely a single sentence. The knowledge we have of them derives from accounts of early philosophers, such as Aristotle's Physics and Metaphysics, The Opinions of the Physicists by Aristotle's pupil Theophratus, and Simplicius, a Neoplatonist who compiled existing quotes.
The first group of Presocratic philosophers were from Ionia. The Ionian philosophers sought the material principle (archê) of things, and the mode of their origin and disappearance. Thales of Miletus (about 640 BCE) is reputed the father of Greek philosophy. He declared water to be the basis of all things. Next came Anaximander of Miletus (about 611-547 BCE), the first writer on philosophy. He assumed as the first principle an undefined, unlimited substance (to apeiron) itself without qualities, out of which the primary opposites, hot and cold, moist and dry, became differentiated. His countryman and younger contemporary, Anaximenes, took for his principle air, conceiving it as modified, by thickening and thinning, into fire, wind, clouds, water, and earth. Heraclitus of Ephesus (about 535-475 BCE) assumed as the principle of substance aetherial fire. From fire all things originate, and return to it again by a never-resting process of development. All things, therefore, are in a perpetual flux. However, this perpetual flux is structured by logos-- which most basically means 'word,' but can also designate 'argument,' 'logic,' or 'reason' more generally. The logos which structures the human soul mirrors the logos which structures the ever-changing processes of the universe.
Philosophy was first brought into connection with practical life by Pythagoras of Samos (about 582-504 BCE), from whom it received its name: "the love of wisdom". 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, including Damon, especially in Lower Italy.
That country was also the home of Eleatic doctrine of the One, called after the town of Elea, the headquarters of the school. It was founded by Xenophanes of Colophon (born about 570 BCE), the father of pantheism, who declared God to be the eternal unity, permeating the universe, and governing it by his thought. His great disciple, Parmenides of Elea (born about 511), 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 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.
Empedocles of Agrigentum (born 492 BCE) 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. the four elements, earth, water, air, and fire. Of these the world is built up, by the agency of two ideal principles as motive forces -- viz., love as the cause of union, strife as the cause of separation. Empedocles was also the first person to propound an evolutionary account of the development of species.
Anaxagoras of Clazomenae (born about 500 BCE) 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, qualitatively distinguished, 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, in which city it reached its highest development, and continued to have its home for one thousand years without intermission.
The first explicitly materialistic system was formed by Leucippus (fifth century BCE) and his pupil Democritus of Abdera (born about 460 BCE). This was the doctrine of atoms -- literally 'uncuttables' -- 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 efforts of all these earlier philosophers had been directed somewhat exclusively to the investigation of the ultimate basis and essential nature of the external world. Hence their conceptions of human knowledge, arising out of their theories as to the constitution of things, had been no less various. The Eleatics, for example, had been compelled to deny that senses give one any access to the truth, since to the world of sense, with its multitude and change, they allowed only a phenomenal existence. However, reason can give one knowledge of what the One is like--or, more accurately, what it is not like.
Retaining the skepticism of the Eleatics about the senses, while rejecting their doctrines about the ability of reason to reach truth apart from the senses, the Sophists held that all thought rests solely on the apprehensions of these 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 for at that time for Greek education. Prominent Sophists include Protagoras, Gorgias, Hippias, and Prodicus.