Heraclitus was called the 'obscure philosopher' because of his often cryptic style.

For him the essential substance that unites all things was fire. He wrote that the world order was an 'ever-living fire kindling in measures and being extinguished by measures'.

He taught that all things are in eternal flux, because of opposites and that 'reality' becomes 'harmony'. He explained his doctrine of eternal change with the remark that 'one cannot step twice into the same river'.

For Heraclitus the first principle of the world was not 'being' but 'becoming'.