Everything Changes, and That Is the Order
Heraclitus of Ephesus wrote in deliberately compressed aphorisms and was called "the obscure" in antiquity. The obscurity is a method: he thought the truth was structured like a riddle, and that stating it plainly would falsify it.
The doctrine everyone knows is flux. "You cannot step twice into the same river", the waters have moved on, and yet it remains the same river. This is not the trivial observation that things change. The claim is that change is what constitutes stability: the river is a river because it flows. A river that stopped would be a lake. Persistence is a pattern maintained through change, not an exemption from it.
The second doctrine is the unity of opposites. "The road up and the road down are one and the same." Opposites are not two things but one thing described from two positions. Sea water is life to fish and death to men; the same road climbs or descends depending on where you stand. Opposites are not merely compatible, they require each other, since neither term has meaning without the other.
Underlying both is the logos: a word carrying "word", "account", "ratio" and "reason" simultaneously. The logos is the rational structure governing all change: the flux is not chaos but ordered, and the order is intelligible. Heraclitus complains bitterly that the logos is public and available to everyone, and that people live "as though asleep", each in a private world.
Fire is his chosen archē, and the choice is exact rather than arbitrary. A flame is a process: it persists as a stable form only by continuously consuming and transforming. It is the perfect image of something whose being consists in becoming, which no static substance could be.
The tension with Parmenides, who denied change altogether, defines the next stage of Greek philosophy, and Plato's theory of Forms is in part an attempt to satisfy both.
Common pitfall: reading Heraclitus as saying nothing is stable. He says the opposite: stability is real, and is achieved through change rather than despite it. The river persists precisely because its water does not.