Democritus (c. 460 - c. 370 BCE) was a Greek philosopher and younger contemporary of Socrates, born in Abdera (though other sources cite Miletus) who, with his teacher Leucippus, was the first to propose an atomic universe. Very little is known of Leucippus and none of his work has survived but he is known by ancient writers as Democritus' teacher, and apparently wrote on many subjects besides atomism.
Known as the 'laughing philosopher' because of the importance he placed on 'cheerfulness', Democritus was the first philosopher to posit that what we refer to as the 'Milky Way' was the light of stars reaching our perception and that the universe may in fact be a multi-verse with other planets sustaining life (a theory which Physicists today are increasingly recognizing as mathematically probable).
In response to Parmenides' claim that change is impossible and all is One, Democritus, among others, tried to find a way to show how change and motion can be while still maintaining the unity of the physical world. With Leucippus, Democritus argued that the world, including human beings, is composed of very small particles which he called 'atomos' (“un-cutables” in Greek) and that these atoms make up everything we see and are. When we are born, our atoms are held together by a body-shape with a soul inside, also composed of atoms and, while we live, we perceive all that we do by an apprehension of atoms outside of the body being received and interpreted by the soul inside of the body. So when atoms have been combined into one certain form we look at that form and say “That is a book” and when they have been combined in another we say, “That is a tree” but, however these atoms combine, they are all One, 'un-cutable', and indestructible. When we die our body-shape loses energy and our atoms disperse as there is no longer a soul inside the corpse to generate the heat which holds the body-shape atoms together.
According to Aristotle, Democritus claimed the soul was composed of fire-atoms while the body was of earth-atoms and the earth-atoms needed the energy of the fire for cohesion. Still, Aristotle also asserts, this did not mean these atoms were different atoms, rather that they were like letters of the alphabet which, though they are all letters, stand for different sounds and, combined in various ways, spell different words. To use a very simple example, the letters 'N', 'D' 'A' can be combined to spell the word 'and' or, with a different combination, spell the name 'Dan' which, while it has a different and distinct meaning from 'and' is still made up of the same letters.
Though there have been some claims made by materialists that Democritus' atomic view of human life denies the possibility of an afterlife, this is not necessarily true. As Democritus seems to have viewed the soul as the causing motion and 'life' and that 'thought' was the physical movement of indestructible, 'un-cutable' atoms, it is possible such a soul would survive bodily death.
The famous line by Leucippus that “Nothing happens at random; everything happens out of reason and by necessity” is a thought which informs a great deal of Democritus' own writing especially his claim that “Everything happens according to necessity” in that atoms operate in one certain way and so, of course, that which happens in life does so out of the necessity of this operation. While this claim would seem to deny the possibility of human free-will, Democritus wrote extensively on Ethics and clearly believed one could make free-will choices within the parameters of atomic determinism.