The ancient Greek philosopher Democritus is credited with first proposing the idea that matter is made up of indivisible particles. He called these particles atoms, from the Greek word "atomos" meaning "uncuttable" or "indivisible."

However, it's important to note that Democritus's idea was based on philosophical reasoning and not experimental evidence. It wasn't until much later, in the 19th century, that John Dalton developed the first scientific atomic theory, which was based on experimental observations and provided a more concrete and detailed model of the atom.