It seems that in the early days of Grecian philosophy, there was a tradition to posit what is known as an arche, translated variously as origin, beginning or first principle. The arche, as I understand it, is the fundamental "stuff" in the sense that all things that exist are simply different manifestations of this primordial substance.

The greek philosopher Thales thought the arche was water. Anaximander, Thales' successor and pupil rejected this idea because of the prevailing idea that water and fire were opposites and so water couldn't give rise to fire.

Heraclitus of Epheseus, another Greek philosopher, was of the opinion that fire was the arche.

There were other theories regarding the arche but as far as this thread is concerned, I want to focus on Heraclitus because I feel his fire is the arche makes complete sense.

Remember, as noted above, Anaximander thought water couldn't produce fire as they were opposites. However, with a little help from modern chemistry, we understand that fire is basically oxidation; fire is different chemicals reacting with oxygen. Given that's the case, what is water? Water, otherwise known as H2O, is simply oxidize Hydrogen i.e. when Hydrogen burns or is on fire we get water. In other words, fire can produce water.

Since water and fire are opposites of each other, everything else must lie between the two and because water is nothing but burnt Hydrogen, it seems that Hercaclitus wasn't too far from the truth if not right on the button about fire being the arche.