We can trace the general concept of the atomic structure of matter back to ancient Greece. In the fifth century B.C.E., the Greek philosopher Leucippus and his famous pupil Democritus (c. 460–c. 370 B.C.E.) introduced the theory of atomism within a society that assumed the world consisted of four basic elements: air, fire, water, and earth. In founding the school of atomism, they speculated that all matter consisted of an infinite number of minute, indivisible particles. Democritus pursued this interesting line of thought much further than his mentor. As a natural philosopher, but not an experimenter, Democritus considered what would happen if he kept cutting a piece of matter, any piece of matter, into finer and finer halves. He reasoned that he would eventually reach the point where further division would be impossible. So he called these final indivisible pieces atoma. Thus the modern word atom comes to us from Democritus and the ancient Greek word atomos (ατοµος), which means “not divisible.” For Democritus, these tiny indivisible particles always existed and could never be destroyed. Different substances resulted from the way they connected or linked together. Like those of the majority of his contemporary Greek natural philosophers, the concepts in Democritus’s school of atomism resulted from hypothesis and the exercise of logic but did not emerge from the rigorous experimentation and observation characteristic of the scientific method. However, we should not treat Democritus too harshly from the perspective of twenty-first century science. About 2,500 years ago his ideas were genuinely innovative and represented the beginning of atomic theory.