Ordinary matter is composed of atoms, themselves once thought to be indivisible elementary particles. The name atom comes from the Ancient Greek word ἄτομος (atomos) which means indivisible or uncuttable. Despite the theories about atoms that had existed for thousands of years the factual existence of atoms remained controversial until 1905. In that year Albert Einstein published his paper on Brownian motion, putting to rest theories that had regarded molecules as mathematical illusions and asserting that matter was ultimately composed of various concentrations of energy.