Aristotle disagreed with Democritus' theory. He was also a philosopher, not a scientist. He believed you could understand and figure out things by simply thinking about them. He also believed that everything was a combination of the four elements: earth, fire, water, air. His theory was that a mass of incomprehensible size was everywhere; he called this 'hyle'. There was no separate 'particles' for each material, it was all one.
His ideas were not contradicted (and agreed with) by anyone until the 17th century. He greatly slowed down the evolution of the atomic theory by saying it didn't exist.
He taught Alexander The Great.