Democritus is credited with developing the concept of atomism. He proposed that all matter is made up of physically indivisible particles called atoms.

Here's a breakdown of Democritus' ideas:

* All matter is composed of atoms: He believed that everything in the universe, from rocks to humans, is made up of tiny, solid particles that cannot be further divided.

* Atoms are eternal and unchanging: He thought that atoms were indestructible and existed in an infinite variety of shapes and sizes.

* Changes in matter are due to changes in the arrangement of atoms: Democritus believed that when matter changed, it wasn't because the atoms themselves changed, but because they rearranged themselves into different configurations.

Important Note: While Democritus' ideas were brilliant, they were based on philosophical reasoning, not scientific experiments. It wasn't until much later, with the work of John Dalton and other scientists, that atomism was truly validated as a scientific theory.