Leucippus and Democritus
Atomism (from ancient Greek atomos, meaning "uncuttable") is a natural philosophy that developed in several ancient traditions. The atomists theorized that the natural world consists of two fundamental parts: indivisible atoms and empty void. References to the concept of atoms date back to ancient India and ancient Greece. In India the Jain, Ajivika and Carvaka schools of atomism may date back to the 6th century BCE. The Nyaya and Vaisheshika schools later developed theories on how atoms combined into more complex objects. In the West, the references to atoms emerged in the 5th century BCE with Leucippus, and Democritus. Whether Indian culture influenced Greek or vice versa or whether both evolved independently is a matter of dispute.
The philosophers Leucippus and Democritus were the first to theorize that the world was composed of tiny particles called atoms. These particles were invisible to the human eye yet ubiquitous in their myriad combinations, comprising what is commonly called reality. Between the two, Democritus was apparently the one with the better sense of humor, because he was nicknamed “The Laughing Philosopher” and “the mocker.” He was allegedly never without a quip or a cackle at the expense of his fellow citizens.
Democritus built on the theories of Leucippus by suggesting that atoms were indivisible. This was accepted as fact until August 1945, when mankind split the atom and unleashed a conflagration, changing the world forever. And quantum physics has proven that there are things even smaller than the atom. But this theory had a good several millennia's worth of fashionability.
The atomistic theory began as an endeavor to overcome the odd logical consequences of the Eleatic school. Leucippus and Democritus did not accept the Eleatic hypothesis that "everything is one" and that change and motion is an illusion. Parmenides had said the void is a fiction, because saying the void exists would mean to say there is something that is nothing, which he thought is a contradiction in itself, but he was deceived by thinking of "being" in the sense of "material being". Thinking of the void as real would have overthrown Parmenides' theory, because allowing the void to exist as "space bereft of body" (Aristotle) with adjoining plenums implies the opposite of classical monism.