With the work of Leucippus and Democritus ancient Greek philosophy reaches its zenith when the initial question of Thales after the true nature of matter culminated 180 years later in the subtle concept of atoms, which bears an amazing resemblance to the twentieth century's view of chemistry. For this reason, Leucippus and Democritus have undoubtedly deserved the first price for the best guess in antiquity, as far as natural science is concerned. Unfortunately their contemporaries did not share their views with the same enthusiasm.
Leucippus is a very shadowy figure; his exact dates are unknown, some even say he never existed, but it is likely that he was a contemporary of Empedocles (around 440 BC) and that he came either from Miletus or from Elea. Democritus, who was a disciple of Leucippus, is a more certain figure. He was born 460 BC in Abdera in the north of Greece and died at the age of 90 years, after leaving an expansive work elaborating his philosophy including the atomistic theory in great detail. Democritus has written approximately 70 books and hence overshadows his master by far. Unfortunately none of his writings remained intact, but a great deal of what he said has survived in Epicurus.
The atomistic theory began as an endeavor to overcome the odd logical consequences of the Eleatic school. Leucippus and Democritus did not accept the Eleatic hypothesis that "everything is one" and that change and motion is an illusion. Parmenides had said the void is a fiction, because saying the void exists would mean to say there is something that is nothing, which he thought is a contradiction in itself, but he was deceived by thinking of "being" in the sense of "material being". Thinking of the void as real would have overthrown Parmenides' theory, because allowing the void to exist as "space bereft of body" (Aristotle) with adjoining plenums implies the opposite of classical monism.
Overthrowing monism was exactly what Leucippus and Democritus intended. They succeeded elegantly by inventing the concept of atoms, for which they are still known. Democritus began with stating a notion of space that served as its premise. Rather than an attribute of matter that describes its extension, Democritus characterizes space as a receptacle for stationary and moving objects, which -under certain circumstances- can as well be completely empty.
Twenty centuries later, Sir Isaac Newton had set forth the receptacle standpoint from where he developed his mechanics. He had a bitter controversy with Leibniz who held, on somewhat different grounds than Parmenides, that space is a system of relations. Today, we realize that both views about space were inaccurate because space can be without solid matter, but it always contains some form of radiation. We also know that the geometry of space is defined by mass, hence, the concept of space as a property of "what is" is closer to the understanding of contemporary physics, therefore Newton is likely to lose this argument today.
Leucippus and Democritus did not care to refute the Parmenidean paradox about the void, instead they simply ignored it, which proved to be useful, because it let them constructively explain motion and change. Change, they explained, is an observation that does not deceive the senses; change is real, it happens on account of the recombination of more rudimentary substances.
Previous Greek philosophers had already raised this point, but prior to the atomists none of them was able to provide a satisfactory explanation for what "substance" is. It was Leucippus' and Democitus' endeavor to develop a theory that would be consistent with sense perception and -by virtue of logical coherence- not contestable by the Parmenidean arguments.
They held that the nature of things consists of an infinite number of extremely small particles, which they called atoms. Atoms are physically, but not geometrically, indivisible. Democritus described atoms as being indestructible and completely full, i.e. containing no empty space. Because of their indestructibility, atoms are eternal. The notion of the atom itself as an "eternal oneness" may be interpreted as a concession to the Eleatic school.
According to the atomists, nature exists only of two things, namely atoms and the void that surrounds them. Leucippus and Democritus thought that there are many different kinds of atoms, each distinct in shape and size and that all atoms move around in space. Surprisingly they did not deem it necessary to give a reason for the motion of atoms, whereby they avoided the sort of logical mistakes that other philosophers had made. They denied that the motion of atoms is impelled in any way, instead they held that atoms move at random, like in the modern kinetic theory of gases. Democritus illustrated the movement of atoms with an observation he made in nature. He compared it to the movement of motes in a sunbeam when there is no wind.
The moving atoms inevitably collide in space, which in some cases causes them to be deflected like billiard balls, and in other cases, when the shapes of two atoms match in a way that they can interlock, causes them to build clusters upon collision, thereby forming substances which make up the objects of our perception. In this regard, Democritus' idea reveals an interesting parallel to Pythagoras, who said that all things are numbers. Because the characteristics of an atom can be described in numbers, any substance can be expressed as a combination of these numbers.
It is controversial whether the atomists also regarded weight a quality of atoms. It seems they simply neglected weight, although Democritus had mentioned that "the more any indivisible exceeds, the heavier it is". At this point, the atomists entered into what their predecessors had postulated to be the origin of matter, namely water (Thales), air (Anaximenses), fire (Heraclitus) and earth (Empedocles). They said, quite accurately as we know today, that these four elements are not primordial substances, but are composed of atoms like everything else.
Contemporary science has proven the atomists right. The atom concept finally took shape in 20th century's views of physics and chemistry. We know atoms as particles with a small positive nucleus that is surround by clouds of electrons and we know that the size of the entire structure is approximately 1/10,000,000 mm. Of course, the antique notion of atoms seems crude by comparison. The characteristics of being indivisible, indestructible, and massive, which had originally been ascribed atoms, cannot be upheld any longer. Today, we also have a better understanding of the internal structure of atoms, and we know that weight, or better mass, is a significant property of atoms.
Nonetheless, Leucippus and Democritus came closer to the truth than anyone else in the following millennium. They developed a fully mechanistic view of nature in which every material phenomenon is seen a product of the atom collisions. Democritus' theory had no place for the notion of purpose and the intervention of gods in the workings of the world. He even held that mind and soul is formed by the movement of atoms. In this regard, his attitude was genuinely materialistic.
Unsurprisingly these views earned Democritus harsh criticism. At a time when orphic beliefs and superstitions dominated the spiritual world, Democritus' atom theory seemed odd. People clung to the belief that their fate was steered by the gods of the Olympus. They were highly uncomfortable with the idea that everything, including human existence, is a product of mere atom collisions. Contemporaries and successors objected that the atomistic theory would leave everything to chance. Plato, for example, does not mention Democritus at all in his works. It is said that he disliked his ideas so much that he wished to see all of his books burned, although it is controversial whether these were his own words.
After Leucippus and Democritus, philosophy made a major turn towards ethics and politics. The atomists were the last in the line of true natural philosophers whose primary subject was the composition and order of the physical universe.
The Presocratic period ended with Democritus. Athens had become the political, cultural and spiritual center of Greece, preparing the ground for the philosophical giants, Socrates, Plato, and Aristotle whose works outshone the atomists for many centuries. Yet, the atom theory remains one of the most amazing intellectual accomplishments of the antiquity.