In the 5th century BC, Leucippus and his pupil Democritus proposed that all matter was composed of small indivisible particles which they called "atoms". Nothing whatsoever is known about Leucippus except that he was the teacher of Democritus. Democritus, by contrast, wrote prolifically, producing over eighty known treatises, none of which have survived to the present day complete. However, a massive number of fragments and quotations of his writings have survived. These are the main source of information on his teachings about atoms. Democritus's argument for the existence of atoms hinged on the idea that it is impossible to keep dividing matter infinitely - and that matter must therefore be made up of extremely tiny particles. The atomistic theory aimed to remove the "distinction which the Eleatic school drew between the Absolute, or the only real existence, and the world of change around us."
Democritus believed that atoms are too small for human senses to detect, that they are infinitely many, that they come in infinitely many varieties, and that they have always existed. They float in a vacuum, which Democritus called the  "void", and they vary in form, order, and posture. Some atoms, he maintained, are convex, others concave, some shaped like hooks, and others like  eyes. They are constantly moving and colliding into each other. Democritus wrote that atoms and void are the only things that exist and that all other things are merely said to exist by social convention. The objects humans see in everyday life are composed of many atoms united by random collisions and their forms and materials are determined by what kinds of atom make them up. Likewise, human perceptions are caused by atoms as well.  Bitterness is caused by small, angular, jagged atoms passing across the tongue; whereas sweetness is caused by larger, smoother, more rounded atoms passing across the tongue.
Previously, Parmenides had denied the existence of motion, change and void. He believed all existence to be a single, all-encompassing and unchanging mass (a concept known as monism), and that change and motion were mere illusions. He explicitly rejected sensory experience as the path to an understanding of the universe and instead used purely abstract reasoning. He believed there is no such thing as void, equating it with non-being. This in turn meant that motion is impossible, because there is no void to move into. Parmenides doesn't mention or explicitly deny the existence of the void, stating instead that what is not does not exist. He also wrote all that  must be an indivisible unity, for if it were manifold, then there would have to be a void that could divide it. Finally, he stated that the all encompassing Unity is unchanging, for the Unity already encompasses all that is and can be.
Democritus accepted most of Parmenides' arguments, except for the idea that change is an illusion. He believed change was real, and if it was not then at least the illusion had to be explained. He thus supported the concept of void, and stated that the universe is made up of many Parmenidean entities that move around in the void. The void is infinite and provides the space in which the atoms can pack or scatter differently. The different possible packings and scatterings within the void make up the shifting outlines and bulk of the objects that organisms feel, see, eat, hear, smell, and taste. While organisms may feel hot or cold, hot and cold actually have no real existence. They are simply sensations produced in organisms by the different packings and scatterings of the atoms in the void that compose the object that organisms sense as being "hot" or "cold".
The work of Democritus survives only in secondhand reports, some of which are unreliable or conflicting. Much of the best evidence of Democritus theory of atomism is reported by Aristotle (384–322 BCE) in his discussions of Democritus and Plato's contrasting views on the types of indivisibles composing the natural world.