Final answer:
Democritus was the philosopher who originally postulated that all matter is composed of indivisible particles, which he called 'atomos.'
Step-by-step explanation:
The philosopher who first argued that all matter is made up of particles so small that they could not possibly be divided was Democritus. He proposed a theory about the existence of atomos, or atoms, which means “indivisible” in Greek. This concept of the atom laid the groundwork for our current understanding of matter, even though it would be centuries before science could prove the existence of atoms. Democritus' fellow Greeks, including Aristotle, held different views, believing that matter could be infinitely divided and composed of different combinations of earth, air, fire, and water.