Within Ionian natural philosophy, one of the important ancient systems of thought was created, atomic theory. It can be summarized as "in reality there is nothing else than atoms and the void." Leucippus from Miletus is regarded as the founder of atomic doctrine. It was further developed by Democritus (ca. 460-370 BC), who was born in Abdera (Thrace) but lived a long time in Athens.
According to atomic theory, the ultimate element so eagerly sought by Ionian philosophers was not a continuous substance, but instead, very tiny, indivisible, and extremely hard bodies, atoms (in Greek: indivisible). When taken alone these atoms lack sensible properties like color, smell, and taste, but they may join together to form all kinds of material things. Leucippus suggested that worlds, which are unlimited in number, arise when atoms fall from infinity into the void and meet each other forming a vortex. In our special case, the Earth collected in the center of such a vortex.
Atomic theory seems to us rather familiar and we may be inclined to view ancient atomists as soul mates of today's scientists. But even more important than the superficial similarity is the realization by the early atomists that the phenomena of the sensible "macro" world may be explained by referring to invisible atoms of the "micro" world. The way they inferred from the visible to the invisible was quite similar to what we do in modern science (even though their detailed explanations went often wrong). Clothes hung out to dry offer a good example of how atomists explained visible things. Wet clothes dry in the sun, but we cannot see the moisture leaving them, because it is split up into minute parts.
It was a key element in the worldview of atomists that bodies were formed quite haphazardly from atoms rushing through empty space. There was no purpose or superior intelligence behind all this. Infinite space and endless time guarantee that sooner or later atoms collide to form whole worlds, of which ours is only one example. Since human beings are made of atoms, and so are our souls that fade away when we die - only the eternal atoms remain. On the basis of these materialistic notions, Epicurus (341-270 BC) from the island of Samos created a view of the world and life which attracted many followers. His ardent Roman admirer, Lucretius (ca. 98-55 BC) later wrote an extensive poem De Rerum Natura (On the Nature of Things) where he describes Epicureanism. Its poetic language contains plenty of information on how atoms were thought to explain natural phenomena and the origin of human sensations. At the same time the poem reflects the enthusiasm with which some people accepted rationalistic thinking about nature - it was seen as a way to disperse the fear of the supernatural.
The world view of the atomists differed radically from the views held by Plato and Aristotle which we will encounter below. For the atomists, the random collisions by atoms were the only "law of nature." Similarly to Anaxagoras, the atomists stripped celestial bodies of their divine nature. However, one must say that their achievements in astronomy were not impressive - for example, Democritus still believed that the Earth is flat and Epicurus was not interested in explaining celestial phenomena. It is slightly ironic that an important step in the development of astronomy into an exact science was made by Plato who believed in the divine nature of celestial bodies. The point is that he viewed the regular movements in the sky as controlled by a superior intelligence and therefore being within reach of a rational explanation.
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