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The Challenge
The ancient Greek Parmenides set his fellow philosophers a conundrum: Since things appear to change (seasons, falling rocks, the cycle of life and death), all that change has to come from somewhere. However, the universe was once a big empty nothing -- so how do you change nothing to something? Parmenides' conclusion: You can't. So, change is just an illusion. Early atomic theorists challenged that rather bleak worldview with a more rational one of their own: The universe consists of some basic underlying material components. These elements can be rearranged to form everything we see today.
Democritus
Democritus argued that the natural world consists of two things: atoms and the void. Atoms, according to Democritus, were tiny but infinitely numerous particles, rocketing around in the equally infinite void. Atoms themselves were changeless and indestructible and couldn't be chopped any smaller (hence their name, from the Greek word for "indivisible"). However, atoms did bump up against one another from time to time, and sometimes they stuck together. All the material things we see -- such as fish, plants, mountains, people -- are collections of these atoms. Since there was no limit on the arena in which the atoms bounced around -- infinite void, remember -- those clusters of atoms could break apart, bounce around and form new things. Hence, the atomic theory: constant change caused by the action of tiny but unchanging building blocks.
The Role of Chance
Critics asked: If we're all composed of tiny bits moving at random, why do the parts of birds, people and the (not yet deteriorated) Parthenon fit so nicely together? Democritus replied that there was really an order to his chaos. Just as birds of a feather flock together, similar atoms liked to get chummy, forming objects of distinctive sizes and shapes. Thus, you'd find olive atoms, but no bird or people atoms, in olives of just the right size. Seagull or eagle atoms clustered to create seagulls or eagles of just the proper size and shape. When people constructed buildings, they were just copying on a macroscopic (or visible to the naked eye) level what atoms did by themselves on a microscopic level. As for the differences in the world we see, the combinations possible from even a small number of atoms make those differences perfectly plausible. Think of how many words can be formed from just the letters of the alphabet.
Thought
All and good, said skeptical philosophers, but what about the soul and emotions and human thought (including philosophy)? More atoms, said Democritus, such as fire atoms in the case of the soul (hence, warm bodies when people are alive and cold cadavers when they're dead and the soul departs). We may smile at these ideas today, but modern researchers looking for a physical basis for human thought in the brain's wiring owe at least a nod to Democritus and his atomic theory.