What made them different
Not their conclusions, most of which were wrong. Their method.
Before them, when you asked why there's thunder, the answer named a god. The Presocratics started answering with impersonal, general principles that applied everywhere and could be argued about. Thales said everything comes from water, which is false and is the right kind of wrong: a claim about nature, defensible by observation, and refutable.
The term is a bit misleading, incidentally. Several of them overlapped with Socrates or outlived him. It marks a style rather than a date.
| Thinker | Roughly | The claim |
|---|---|---|
| Thales | 624-545 BCE | Everything is fundamentally water. First recorded attempt at a natural explanation of everything. |
| Anaximander | 610-546 BCE | The origin is the apeiron, the boundless: indefinite rather than any known substance. A remarkable abstraction. |
| Anaximenes | 586-526 BCE | Air, condensing and rarefying into everything else. First real mechanism for change. |
| Pythagoras | 570-495 BCE | Number is the structure of reality. Also a religious cult with rules about beans. |
| Heraclitus | 535-475 BCE | Everything is process; opposites are one; fire is the model. Deliberately obscure. |
| Parmenides | 515-450 BCE | Change is impossible and the senses lie. Reasoned to a conclusion he knew was absurd and refused to flinch. |
| Zeno of Elea | 490-430 BCE | Parmenides' defender. The paradoxes: Achilles and the tortoise, the arrow. |
| Empedocles | 494-434 BCE | Four elements, driven together and apart by Love and Strife. Reportedly jumped into Etna. |
| Anaxagoras | 500-428 BCE | Mind (nous) orders everything. Tried for impiety for saying the sun was a hot rock. |
| Democritus | 460-370 BCE | Atoms and void, and nothing else. The closest ancient guess to modern physics. |
Parmenides is the one to notice
He argued that what is, is; what is not, cannot be; and therefore nothing can come into being or pass away, because that would require something to come from nothing. Change is therefore an illusion, and the senses that report it are unreliable.
He knew this contradicted every waking moment of experience. He followed the argument anyway.
That's the moment reason is set against observation as a source of truth, and Western philosophy spends the next 2,500 years dealing with it. Plato's forms, the atomists, and arguably the whole rationalist tradition are responses to Parmenides.
How we have any of this
Fragments, quoted by later writers with their own agendas. Aristotle is a major source and he summarises his predecessors as steps towards his own position, which is not a neutral procedure. Simplicius, writing a thousand years later, preserved long passages simply because he was thorough.
So we're reading a heavily filtered selection, chosen mostly by people who wanted to argue with it.
Where to read them
Kirk, Raven and Schofield's The Presocratic Philosophers is the scholarly standard. Robin Waterfield's The First Philosophers (Oxford World's Classics) is the readable one-volume option and the right first buy.
See also Socrates, who turned the subject from nature to how to live, and 365 Days of Philosophy, which opens with Thales on 1 January and works forward.
Common questions
Who were the Presocratic philosophers?
Greek thinkers from roughly 600 to 400 BCE who tried to explain the world through general natural principles instead of gods, including Thales, Anaximander, Heraclitus, Parmenides, Empedocles and Democritus.
Why are the Presocratics important if they were wrong?
Because the method mattered more than the answers. They made claims about nature that could be argued about and refuted, which is the move that starts science and philosophy.
Did all the Presocratics come before Socrates?
No. Several overlapped with him or outlived him. The label marks a style of enquiry rather than a date.
What did Parmenides argue?
That nothing can come from nothing, so change and coming-to-be are impossible and the senses mislead. He accepted the conclusion despite it contradicting all experience, and Western philosophy has been answering him since.
What's the best book on the Presocratics?
Robin Waterfield's The First Philosophers for a readable single volume, and Kirk, Raven and Schofield for the scholarly standard.