The concept of the atom (Greek: atomos, "indivisible"), an indivisible particle of matter, goes back to ancient Greece and a man named Democritus, a rival of Aristotle. Democritus held that all matter could be subdivided only until some finite particle was reached. Aristotle had a quite different idea, that matter was a continuous substance, not composed of any fundamental units. And Aristotle was much more famous and admired in his time and so his error persisted into the late 1700's. But eventually work by men such as Lavoisier began to suggest that Aristotle had been seriously wrong.
In 1808 the first statement of a modern atomic theory was published by John Dalton, a Quaker schoolmaster from Manchester.
It may not seem like much, but such a theory was used to explain two of the major laws in chemistry: the Law of Conservation of Mass and the Law of Constant (definite) Composition. And the third postulate led Dalton to formulate the Law of Multiple Proportions.