The concept of matter being composed of tiny, indivisible particles dates back to ancient Greek philosophers, but the specific credit for proposing it is generally given to Democritus.

While other philosophers, such as Leucippus, may have contributed to the idea, Democritus is recognized as the main proponent of the atomic theory. He proposed that all matter is made up of physically indivisible particles called "atoms" (from the Greek word "atomos" meaning "uncuttable"). He believed these atoms were eternal and unchanging, and all changes in matter resulted from the rearrangement of these atoms.

However, it's important to note that Democritus's theory was based on philosophical reasoning and not scientific experimentation. It wasn't until much later, in the 19th century, that the atomic theory was scientifically validated by John Dalton and others.