The most striking characteristic of this first phase of Greek philosophy is its preoccupation with such questions as the origin of the physical world; the nature and substance from which it evolved; the form of its main components (the earth and the heavenly bodies); the explanation of such phenomena as rain fall, storms, lightning, and thunder; the phases of the moon; the eclipses of the sun and moon; the formation of the sea ; and the origin of animal and human life. Beginning with the 6th century B.C. and continuing through the 5th, a series of diverse cosmological schemes were offered in rapid succession, many of them works of astonishing imaginative audacity and constructive power. The most important of them are conveniently identified by their respective theories of the primordial stuff of stuffs: water for Thales; the "Infinite" for Anaximander; air for Anaximenes; fire and earth for Parmenides; fire, air, water, and earth for Empedocles; an infinite number of qualitatively different stuffs or "seeds" for Anaxagoras; and an infinite number of atoms, qualitatively homogeneous, distinguished only by size, shape, and motion for Anaxagoras' contemporary, Leucippus, and also for Democritus. For all the recklessness of their speculations and the unscientific dogmatism of their temper, these thinkers propounded a concept of nature that is indispensable for all scientific thought; they thought of nature as a system of events exhibiting an absolutely dependable, unexceptionable causal order. They expunged, for the first time in western thought, the notions of miracle and magic from their picture of the world; they abolished the supernatural.
This imaginative revolution was bound to have a shattering effect on religious belief. Its implications were fatal for the anthropomorphic gods of the popular faith, as Xenophanes made clear in satirical jabs like, "if oxen and horses and lions had hands, the horses would paint their gods in horse-like form, the oxen in ox-like form, they would give the gods bodies like their very own."
But the mood of Xenophanes, and of the pre-Socratics generally, was not that of the village atheist. Reverent before the majesty of the natural order, they were prone to impute its source to a cosmic intelligence (Anaximander, Xenophanes, Heraclitus, Anaxagoras). Yet none of them thought of this reason that "governs" the world as an immaterial spirit. That whatever exists is corporeal remained a widespread assumption in pre-Socratic thought.
Pythagoras played a religious and political role unlike that of any other pre-Socratic. He founded a school that acquired great political influence in the Greek states of southern Italy. Its discipline was like that of a religious order, and its main dogma was the reincarnation of the soul.
It made important advances in mathematics, harmonics, and astronomy. It adhered to the strange doctrine that "all things are numbers." Heraclitus expounded his philosophy in paradoxical aphorisms, masterpieces of terse, reverberant, evocative prose. Their main message is that the world is in constant flux but retains eternal permanence of form in virtue of the fact that contrary changes constantly balance. Here cosmology becomes subordinate to distinctively philosophical theses, like the ubiquity of change, the "identity" of opposites, the contrast between the one common world of law and reason, on one hand, and the many isolated worlds of private dream and whim, on the other.
Parmenides, modeling philosophical method on mathematical deduction, professed to demonstrate the logical impossibility of change and to establish that "Being" (all that exists) is ungenerated, indestructible, motionless, continuous, homogeneous, one. These conclusions, so defiant of sense experience and common sense, were defended by his pupil Zeno with arguments that have fascinated philosophers and logicians ever since and are still actively debated today. Though Parmenides found few disciples, he exercised enormous influence on his immediate successors, on the atomists most of all: the Democritean atoms are, like Parmenidean "Being," absolutely Immutable and homogeneous, but their evershifting combinations and permutations account for the ever-changing diversity of the visible world.