Democritus was an ancient Greek philosopher born in 500 BC in the city of Abdera (where the famous sophist Protagoras was also born). He was the student of Leucippus, and together they are referred to as the Atomists because they put forth a theory of the universe made up of infinite number of invisible and indivisible atoms. The Atomic theory was created in response to Parmenides, who denied the possibility of change. Democritus explained that the things we see as changing are the result of atoms combining with one another in different ways. Because of the lengths he went to describe the world naturalistically (i.e. without Gods), materialistically, and mechanistically (“this causes that”), he is seen by many as the “father of modern science.”

Democritus traveled in Egypt and Babylonia, studying along the way. He would write over 70 works on all sorts of topics including natural philosophy, ethics, mathematics, language, medicine, and music. He has been called the “Laughing Philosopher” (in contrast to Heraclitus, the “Weeping Philosopher”) because he believed that cheerfulness is good for the soul. Unfortunately, because he wrote in Abdera rather than Athens, and because influential philosophers to come such as Plato and Aristotle didn’t like his atomic theory, not much of his writings have survived.

Democritus is unique in that he took his teacher Leucippus’ theory and developed it without changing it. His atomic theory states that there are two types of things in the world: atoms and the void. Atoms are infinite, eternal, and indivisible (atamos means “unsplittable”). There are an infinite number of atoms with infinite shapes. His understanding that there are infinite atoms allows him to understand all the things in the world, and in this way he presents a microscopic theory to explain the macroscopic natural world (i.e. how things look to the naked eye).

Atoms do not have perceptible qualities (like color or texture). What happens is that atoms move around in space surrounded by the void and bump into each other like billiard balls. In this process, atoms come together to create compounds. While the atoms themselves are eternal, indivisible and without qualities, the compounds that are formed have perceptible qualities. For example, a spherical atom can combine with other atoms to create fire, or it can combine with different atoms to create soul. These compounds exist until other atoms come along and knock the atoms making up the compound out of alignment. In this way, Democritus’ atomic theory is able to account for all the things of the world as well as make sense of change (for example: water turning into vapor).

Zeno of Elea, the student of Parmenides famous for his paradoxes, believed that all objects could be divided infinitely smaller. This was an issue for him because if something could be divided into infinitely smaller parts, that means the thing itself is made up of an infinite number of things having a certain size. Therefore, the thing as a whole should be of an infinite size rather than a finite size. Alternatively, if the object is divided infinitely to the point where the smallest parts have no size, then the thing would be made up of an infinite number of things without any size. Therefore, the thing as a whole shouldn’t have any size. Either way, it doesn’t make sense. This was a confusing issue which Democritus will avoid altogether, because he didn’t see any necessary reason to believe that things can be divided infinitely. Instead, he put forth his atomic theory which says that things can be divided up to a point, the atom, and no further.

The atomic theory allowed Democritus to respond to the philosophers of Elea (Parmenides and Zeno) who believed that the world must be eternal and One because of the impossibility for something to come from nothing (creation), as well as for something to become nothing (destruction). Democritus was able to agree with them that the world was made up of an eternal substance (the atoms and the void), but because there is an infinite number of each, he was also able to maintain that change is possible, as well as motion, and that the world exists of many things (plurality). Since the atom itself does not change when it combines with other atoms to form compounds, the principle of identity is maintained (i.e. “that which is, is”).

It is their understanding of the void that allows Leucippus and Democritus to disagree with Parmenides. The void allows the atoms to be separated from one another. This separation is necessary for motion, because without being separated, everything would just be one thing (as Parmenides believed). In this way, that which allows Democritus to disagree with Parmenides is his belief that “nothing” does in fact exist. While for Parmenides, “nothing” could not exist because nothing could be said or thought of it, for Democritus “nothing” is the void, exists, and is just as real as the atoms themselves.

Because atoms move out of necessity in Democritus’ atomic theory, their movements are the result of the last atoms they collided with. Neither chance nor purpose exist in the world. In theory, everything makes sense and can be explained. Democritus’ cosmogony (understanding of the world), like other Greek philosopher’s, serves as a radical break with the traditional Greek mythological understanding of the world as being directed by the whims of the Gods. Instead, we find a rational and scientific theory. The lightning bolt is no longer hurled by Zeus, but is explained as the combination of atoms that occurs when clouds suddenly come together. Even the specialness of the world is denied. Democritus’ universe (“the whole”) is made up of many cosmoi (worlds), which form when large amounts of atoms combine, and inevitably will become undone again when the atoms break up.

Democritus used his atomic theory to explain other things such as how humans are able to sense, and otherwise perceive, things. He actually considered the human to be a microcosmos (“small world”). He believed that when we perceive something, atoms from the object strike the atoms in our sense organs, such as the ear or the eye. Thought is also the result of atoms in our bodies being activated. This allowed Democritus to understand the senses as both helpful as well as limited. Through this interaction of atoms from outside and within, we can experience the way things appear to us as compounds. However, since atoms themselves do not have any of the properties that appear to us, true knowledge lies beyond that which we can understand empirically (through the senses).

Democritus also applied his atomic theory to other areas, such as theology. He understood Gods to exist, but saw them as large beings (compounds of atoms) which lasted a long time but were not eternal (since compounds by necessity will eventually break up when hit by other atoms). He also related his atomic theory to ethics (“the best way to live”), by explaining how the arrangement of soul atoms in the body is affected positively by acting in positive ways, and by living a happy life. As he put it, “teachings reshape the person, and by reshaping makes his nature.” He therefore recommended a way of life that focuses on actions of merit, and minimizes excessive pleasure-seeking (over-eating, getting drunk, being promiscuous) that would be embraced by the Epicureans along with his understanding of the world as a whole.

Democritus’ atomic theory was not embraced by everyone. Both Plato and Aristotle opposed it, and while Plato puts forth a similar “geometrical atomism” at the end of his dialogue the Timaeus, his Theory of Forms is the opposite of Democritus’ understanding of the world. This is because Plato’s Forms of the Good, Beauty, and Order give the world a purpose (teleology), while for Democritus the world is understood in simple, mechanical terms.

While the atomic theory didn’t catch on with everyone, in the 18th century it was revived by the French philosopher and scientist Pierre Gassendi and in the 19th century the classical theory of chemistry understood atoms to be the smallest units of matter, coming together to form molecules. Eventually it was discovered that there are smaller things than atoms, such as protons and electrons. But these subatomic particles can be seen as parallel to Democritus’ atoms as the smallest “unsplittable” things. The real difference is that modern atomic theory has found there to also be an immaterial binding force in atoms that Democritus’ did not know about. But, modern science also has powerful microscopes, while the classical atomists only had reason to rely on. While creating an impressive physical theory of the world, after Democritus the focus in Greek philosophy would shift from science (“natural philosophy), to the realm of politics and ethics.