There are some problems with naming the Pre-Socratic philosophers.
The term was invented by Hermann Diels – He thought that the philosophy of Socrates was moral and the philosophies of the Pre-Socratics was physical/cosmological.
Historical chronology is not relevant; many were contemporaries of Socrates.
Most of their texts don’t survive – only fragments and references in other works are available.
Many didn’t consider themselves to be philosophers at all.
Thales of Miletus
Aristotle described him as the founder of natural philosophy.
He thought that the world was made out of water.
The thesis that the Earth is a sphere is attributed to him.
Much of his knowledge came out of astronomy. He predicted a solar eclipse in 585 BCE – one of the first astrological predictions in Western history.
Heraclitus of Ephesus
Instead of focusing on the cosmos, he focused on the human realm.
He thought that fire was the main substance that the world was made out of.
Nothing is stable; everything is in flux, constantly changing.
He claimed that there was a unity of opposites – they could not be separated from each other, but were instead found in the same being. An example of this is sleeping and waking. While we are never asleep or awake at the same time, we are always either awake or asleep – both our present in us, even if only as potential.
These opposites can be in conflict. It is the conflict between opposites that drives change in the world.
The alternative to this view of this world is unchanging uniformity, which Heraclitus claimed could not explain the change in the world.
Parmenides of Elea
He was a critic of Heraclitus.
Different from many Pre-Socratics – he wrote in Homeric Hexameter.
He claimed that change was impossible, since the world would have to be affected from something not in the world; however, whatever is outside of existence is nothing, so it cannot exist.
Therefore, the world is static. The perception of change is false.
We can only know what is true by distinguishing what is (reality) from what is not (appearance). This was an early version of the principle of non-contradiction.
We come to know the world through thought and reflection on the world – logos.
One of the founders of atomist theory.
He claimed that everything in the world is matter or void.
The world is made out of a combination of elements.
The motion and qualities of elements cause sensation. For example, jagged elements cause bitterness when tasted, but they also cause heat from friction between them.
Thought and perception are changes in the body themselves – the mind is not separate from the world.
The Pre-Socratics heavily influenced Plato and Aristotle, as well as the history of philosophy in general.
They helped create an interest in cosmology and astrology.
Parmenidies influenced the principle of non-contradiction – Leibniz and modern logic rely on this principle.
Heraclitus was a main influence on Nietzsche and much of later continental philosophy (Heidegger’s reading of Parmenidies, Deleuze’s metaphysics).