Democritus, a Greek thinker, was first to propose the existence of the atom; it took nearly two thousand years before the atom was successfully described as a fundamental chemical object. This was done by John Dalton (1766-1844), and two hundred years later his theories regarding atomic theory remains valid in the minds of our contemporary physicists. Dalton's theory is comprised of four pieces. First, that all matter is comprised of indivisible, indestructible pieces called atoms. Second, that all atoms of a single element are identical in mass and properties. Third, that compounds are formed by a combination of two or more of these different kinds of atoms. And lastly, that a chemical reaction is a rearrangement of atoms.