The biologist Lewis Wolpert calls it "Thales's leap."
It is to Thales of Miletos, who lived about 600 B. C., that Wolpert gives credit for being the first to suggest that the world is primarily matter, not mind. Thales chose water as the primary substance. A reasonable guess, to be sure: water exists as a solid, a liquid and a gas; it is manifestly essential to life, and hence to mind. Understand water said Thales, and you are on you way to understanding the world.
It was a daring suggestion that ran counter to every prior explanation that we know about, all of which invoked an animate and intelligent creative force. In place of the Ionian pantheon of gods, Zeus and all his cohort, including the spirits of rocks and pools and trees, Thales said "water."
Of course, Thales was himself a creature of his time, and he almost certainly had precedents that we don't know about, but give him his leap. Start with "stuff." Stuff has one great advantage over mind as an explanatory principle: it can be described mathematically, and Thales was no slouch as a mathematician. With the union of stuff and mathematics, science as we know it began.
We have come a long way from water as the primary stuff. We now understand that water molecules are composites of atoms of hydrogen and oxygen, which are composed of electrons and protons. And even these subatomic particles are...
No more hard little particles rattling around in the void, as proposed by Democritus, Lucretius and Newton. Matter, as it shows itself at the turn of the millennium, is a thing of astonishing, almost "immaterial" subtlety, with a built-in capacity to complexify and diversify, to spin out stars and galaxies, hydrogen, oxygen, carbon, and ultimately life and consciousness.
As physicists probe the structure of atoms, the fundamental particles dissolve into a kind of cosmic music, all resonances, vibrations and spooky entanglements. There is nothing at the heart of matter that is quite "material" in the way we previously understood the word, but we only got to this place by taking Thales's leap and committing ourselves to a materialist philosophy.