Heraclitus saw stability as illusion. The river seems the same, but both water and you are continuously changing. This applied to everything: "Cold things warm up, warm things cool down, moist things dry out, dry things get wet." Permanence is merely slow change.
Yet Heraclitus didn't advocate chaos. He proposed that flux itself follows a pattern—the Logos, a rational principle governing change. Fire served as his primary metaphor: transforming everything while maintaining its essential nature, constantly becoming something else while remaining fire.
His doctrine of the unity of opposites held that contraries are necessary to each other: "The way up and the way down are one and the same." Health requires sickness, justice requires injustice, harmony emerges from tension. Opposition doesn't negate unity but constitutes it.
Heraclitus profoundly influenced Western philosophy, from Plato's theory of Forms (a response to flux) to process philosophy and dialectical thinking.
Source
Fragments of Heraclitus