What we like to think of as "philosophic thought" first appears in Greece in a poem, Theogony , written by Hesiod about 725 B.C.; the Theogony retells the myths of the gods and speculates in part about the origins of things and the order of the universe. What we generally call "Greek philosophy" was almost certainly derived by the Greeks from Egyptian culture, particularly natural science (physics and math) which preoccupied Greek thought up to the time of Plato. The Greeks seem also to have derived much of their philosophical theology from the Egyptians as well. These are not modern interpretations of Greek philosophy; the ancient Greeks themselves claim without dissension that their philosophy comes from Egypt. Whether the Greeks travelled to Egypt or whether the Egyptians colonized or visited Greece at some point (which is what the ancient Greeks thought) is a difficult question to answer.
Nonetheless, in the latter half of the fifth century, a group called the Sophists ("those with wisdom") shifted the inquiry away from natural science and towards the nature of morality and society. Socrates follows in the footsteps of the early Sophists in making ethics his primary topic; with this and with Plato's overwhelming concern with ethics, Greek philosophy became primarily concerned with ethical and civic virtue. A knowledge of the ideas of their predecessors is the best way to approach Plato and Aristotle. These early Greek philosophers who preceded Socrates, the Pre-Socratic Greek philosophers, are divided up into several schools.
The Milesian or Ionian School. Miletus, on the coast of Ionia in Asia Minor, in the 6th century was a prosperous trading center with numerous colonies; its government was aristocratic, and no divinely-backed King or priesthood dominated society and learning. In order to explain the origin of things and the nature of change and motion, the philosophers in Miletus sought to discover or describe one primary, material substance as the base or elemental foundation of all natural objects and the source of all motion; Thales (early 6th century) postulated that this primary substance was water, Anaximander defined the primary substance as "the unlimited" or "the indefinite" (in Greek: apeiron ), Anaximenes defined it as air.
The Pythagoreans. Pythagoreanism began towards the end of the 6th century in the Greek cities in southern Italy; this school sought an intellectual foundation for a certain religious way of life, and was more abstract and mathematical than the Milesians (and much more heavily influenced by Egyptian thought). Much of their thought remains completely obscure and impenetrable. They principally sought to purify the soul by strict rules of life; they believed in metempsychosis (the transmigration of souls to animals and even plants); and they found the essential unity of things to lie not in a physical substrate but in number and numerical relations. For the Pythagoreans, the one thing that formed the substrate of all the infinite things in the universe was number .
Later Philosophies. The most important of the later thinkers who developed these early ideas were:
Xenophanes of Colophon (born in Ionia; settled in Southern Italy; lived c. 570-c. 475) ridiculed the anthropomorphic gods of Greece and believed in one great God, which was not physical but was all mind (in Greek: nous ), moving all things by the force of his spirit without himself having to move (since mind was not physical, it couldn't move).
Heraclitus of Ephesus (in Ionia; contemporary with Xenophanes) saw change as the unity of all things; he took movement or the contrary tension of things (such as a taut bow, which is potential movement because of the opposing forces at work) as forms of the mutual resolution of opposites. The unity underlying all change and opposition, but not existing outside of change and opposition, Heraclitus alternatively called the Logos or God. With Parmenides, Heraclitus is perhaps the most important philosopher before Plato, for the idea that nothing transcends change threw a monkey wrench in Greek speculation about physics and metaphysics and has formed the foundation of Western thinking ever since.
Parmenides of Elea (in Southern Italy; active in the first half of the fifth century) founded the Eleatic School and taught that Being (or Existence) must be unchanging and unmoving, and so the changing world registered by our senses has no reality whatsoever and cannot be known at all (how can you "know" an illusion?). Only reason, without the senses, can lead us to the truth about existence, which neither moves nor changes nor has any parts. This is diametrically opposed to Heraclitus's view, and like Heraclitus's thought, the Eleatic school effectively mucked up Greek speculation about the nature of things for quite a few decades. However, the Parmenidean idea of the nature of reality would become the basis of Plato's thinking and would later become the foundation of the Christian theology of God.
Zeno of Elea, a pupil of Parmenides, produced famous paradoxes which were essentially arguments supporting Parmenides's views. Just about all of Parmenides's contemporaries thought his theories a bit kooky and logically impossible. Zeno attempted to show that those people who believed that things move, change, and have discrete parts are the ones subscribing to kooky theories by demonstrating that motion and divisibility were logically impossible. Zeno's best-known paradox is the race between Achilles and the tortoise, in which Achilles may never catch a tortoise if it's given a head start ina race. For before he caught up to the tortoise, Achilles would have to reach a point half-way from his starting point and the tortoise, then he must go half-way again, and so to infinity. No matter where Achilles is in relation to the tortoise, he still has an infinity of half-way points to cross, so he can never catch up to the tortoise.
Empedocles of Acragas (in Sicily; active mid-5th century) tried to reconcile the views of Heraclitus and Parmenides by identifying four basic elements (which become the standard elements up until modernity): earth, water, air, and fire. These elements remain unchanging but combine to form the changing and moving world of our senses.
Anaxagoras (of Athens; perhaps 500-428) taught that all things come to be from the mixing of innumerable tiny particles of all kinds of substance, shaped by a separate, immaterial, creating principle, Nous ("Mind"). Nous is not explicitly called divine, but has the qualities of a creating god; Nous does not create matter, but rather creates the forms that matter assumes.
The Atomists, Leucippus (about whom almost nothing is known) and Democritus (of Abdera, in Thrace, born about 460), held that void (space with no matter) exists (against the Eleatics, who held that what is not there cannot exist) and that this void contains an infinite number of indivisible units (atoma , which means "indivisibles") which are undifferentiated in material but different in size and shape. By random movements they form vortexes, in which similar atoms come together and form the sensible world. This theory was taken over later by the Hellenistic philosopher, Epicurus.
The Sophists. The Sophists were professional teachers who, for a fee, would undertake to teach their students how to get ahead in the world. Socrates was often allied with them by his contemporaries, but his purposes were, in some respects, different. The best-known Sophists were Protagoras, Gorgias, and Hippias, who were significant original thinkers. To get a position of importance, especially in a democracy, one had to have oratorical skill, strength in debate, and a knowledge of law and politics; one would need to know how to manage property and maybe run the State, and know something of music, astronomy, math, physics, and so on. The Sophist equipped one to be a leading citizen, and supplied answers to help people live in a world whose reality had been somewhat undermined by the Pre-Socratics. Philosophers of antiquity have nothing but bad things to say about the Sophists (even though it is extremely likely that Thucydides adhered to Sophistic principles) and we share this attitude. However, despite all we say about the origins of the modern university in Plato and Aristotle, the modern university owes more to Sophistic principles than any other philosophical school. Perhaps the most important Sophistic idea is a brand of skepticism: human affairs, ethics and politics, do not admit of certain knowledge so that good arguments can be made on either side of an argument . This view will dominate later antiquity and modernity and the exercise of arguing on both sides of the question (called in Latin, in utrumqem partem ) is one of the principal aspects of Western education from the Roman Republic to the twentieth century.
A big question that concerned the Sophists and their critics was: how is virtue (areté) acquired? Can it be taught? These aren't ivory tower questions, such as the nature of Nous , but were vital in a society where power was shifting from the "well-bred" aristocrats to the less educated masses. Answers involved two different principles which are essential for understanding Greek thought: physis , "the unchanging," "fundamental existence," or "nature" (we get the word "physics" from this term: physics in the Greek world is primarily the study of "the unchanging" and secondarily the study of "nature") and nomos , "custom," "the changing," "convention," or "law," and includes morality, tradition, and state laws, all of which are subject to change or revision.
This dichotomy is a little like that between "nature" and "nurture." However, what the Greeks meant by physis is a bit different from our idea of "nature" or "natural." Physis designated what remained constant (like the gravitational constant) and so could not be changed; however, sometimes the Classical Greeks use this term to mean something like "nature." From the third century onwards, physis will mean something closer to our idea of "natural law." For the Greeks the scope of the dichotomy between nomos and physis applied to practically everything, and it entered into questions such as: do gods really exist (physis ), or are they only a conventional human belief (nomos )? Are class divisions or gender divisions natural (physis ) or artificial (nomos )? Is justice an inborn characteristic of humanity (physis ), or a convention invented by the weaker as a defense against the stronger (nomos )? Is it natural for the stronger to rule the weaker? Is it "natural," or merely self-evident, for all men to be created equal . . .?
Both nomos and physis may be considered good or bad. Nomos brings progress in society (as in Pericles's Funeral Oration or the American Constitution); but if laws are only valid by nomos they may be changed with circumstances, and may conflict with physis. Physis justifies universal laws ("honor thy father and thy mother") and the equality of rich and poor, men, women, and slaves; but can also justify considering the laws of the State an unnatural limitation on individuals, to be observed only when transgression is likely to be found out. For instance, I could justify breaking the law by appealing to some universal right or some universal law; when governments defend "freedom fighters" rebelling against other governments, they are appealing to some universal, unchanging law that is greater than the laws of an individual state.