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Pre-Socratics

The pre-Socratic philosophers were interested in cosmology; the nature and origin of the universe, while rejecting unargued fables in place for argued theory, I.e., dogma superseded reason, albeit in a rudimentary form. They were specifically interested in the arche (the cause or first principle) of the world. The first recognized philosopher, Thales of Miletus (born c. 625 BCE in Ionia) identified water as the arche (claiming "all is water") His use of observation and reason to derive this conclusion is the reason for distinguishing him as the first philosopher. Thales' student Anaximander claimed that the arche was the apeiron, the infinite. Following both Thales and Anaximander, Anaximenes of Miletus claimed that air was the most suitable candidate.

Pythagoras (born c. 570 BCE), from the island of Samos off the coast of Ionia, later lived in Croton in southern Italy (Magna Graecia). Pythagoreans hold that "all is number", giving formal accounts in contrast to the previous material of the Ionians. The discovery of consonant intervals in music by the group enabled the concept of harmony to be established in philosophy, which suggested that opposites could together give rise to new things. They also believed in metempsychosis, the transmigration of souls, or reincarnation.

Parmenides argued that, unlike the other philosophers who believed the arche was transformed into multiple things, the world must be singular, unchanging and eternal, while anything suggesting the contrary was an illusion. Zeno of Elea formulated his famous paradoxes in order to support Parmenides' views about the illusion of plurality and change (in terms of motion), by demonstrating them to be impossible. An alternative explanation was presented by Heraclitus, who claimed that everything was in flux all the time, famously pointing out that one could not step into the same river twice. Empedocles may have been an associate of both Parmenides and the Pythagoreans. He claimed the arche was in fact composed of multiple sources, giving rise to the model of the four classical elements. These in turn were acted upon by the forces of Love and Strife, creating the mixtures of elements which form the world. Another view of the arche being acted upon by an external force was presented by his older contemporary Anaxagoras, who claimed that nous, the mind, was responsible for that. Leucippus and Democritus proposed atomism as an explanation for the fundamental nature of the universe. Jonathan Barnes called atomism "the culmination of early Greek thought".

In addition to these philosophers, the Sophists comprised teachers of rhetoric who taught students to debate on any side of an issue. While as a group, they held no specific views, in general they promoted subjectivism and relativism. Protagoras, one of the most influential Sophist philosophers, claimed that "man is the measure of all things", suggesting there is no objective truth. This was also applied to issues of ethics, with Prodicus arguing that laws could not be taken seriously because they changed all the time, while Antiphon made the claim that conventional morality should only be followed when in society.