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2.1 Thales and Anaximander
Milesian philosopher Thales’s view that the world is constituted by or originated from water is defended on the grounds that water appears to be elemental.
Anaximander, another Milesian philosopher, is said to be a pupil of Thales. His view of the origin of things derives from the principle of opposites, and is traceable back to the imperishable apeiron. This formless substance is the source of all things.
2.2 Heraclitus
One of two major influences on Plato’s thought, Heraclitus holds that opposites ultimately find harmony in the principle of the logos, which orders the world. The seeming flux of things is just that, a seeming. Underneath, rational and divine logos steers all things.
2.3 Parmenides
The other most significant influence on Plato’s thought, Parmenides, set the stage for later theoretical divisions between reason and sensation. According to Parmenides, reason dictates that reality is one, not many. The many appears, but that is the extent of its reality.
Moreover, the One necessarily is. Parmenides justifies this claim by reasoning that non-being is impossible—one cannot think or speak about non-being, since the thinking and talking is about something. A further consequence is that we cannot speak about change. Hence, “what is, is, and cannot not be.”
2.4 Democritus
Democritus is one of two atomists. Along with Leucippus, Democritus firmly rejects Parmenides’s monism. He holds a pluralistic view of the universe: there are infinitely many atoms and the void. The void is not nothing, but it is also not equivalent to Parmenides’s One. Democritus’s view of the atom does bear similarities to Parmenides’s view of reality, namely that it is indestructible and solid, even if it varies in size and shape from other atoms.
2.5 Protagoras and the Sophists
Protagoras is one of the most famous of the itinerant teachers, the Sophists, whose views and methods Plato—and most of Athens, for that matter—largely reviled. Arguably, Protagoras’s single most important view was moral relativism.