Heraclitus of Ephesus (6th century BC), an Ionian philosopher, believed that fire was the most important element. This contrasts with the views of Thales and Anaximenes, both from nearby Miletus, who favoured water and air respectively.

Several of Heraclitus’ views have aspects in common with our modern understanding, although we should not get carried away with these similarities. He proposed that:

• There was a continual flux of fire that transformed into water and earth. Compare this with the ability to transform energy into matter through Einstein’s famous equation, E= mc²
• There was a simple conservation law (of volume) between the amount of air, water or earth that was created from fire.