Many of the ancient Hellenic philosophers were scientists. We call them philosophers only because they couldn't prove many of their theories. While many of their theories were discredited once the option to truly research these issues were developed, it were their theories that caused later generations to seek this proof. One of these examples is Demokritos' theory on atoms.

Demokritos (Δημόκριτος) was born in Abdera, Thrace, around 460 BC, although, some thought it was 490 BC. His exact contributions are difficult to disentangle from those of his mentor Leucippus, as they are often mentioned together in texts. Largely ignored in ancient Athens, Democritus is said to have been disliked so much by Plato that the latter wished all of his books burned. He was nevertheless well known to his fellow northern-born philosopher Aristotle. Many consider Democritus to be the "father of modern science". None of his writings have survived; only fragments are known from his vast body of work.

The atomic theory of Democritus held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible, and have always been and always will be in motion; that there is an infinite number of atoms and of kinds of atoms, which differ in shape and size. Of the mass of atoms, Democritus said, "The more any indivisible exceeds, the heavier it is". But his exact position on atomic weight is disputed. Materialist philosophers Democritus and Leucippus posed that everything--including humans--existed from atoms from the same source. The way these atoms moved and reacted to each other controlled causal laws.

Demokritos knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole. Therefore, he reasoned that if the stone were to be continually cut into smaller and smaller pieces then; at some point, there would be a piece which would be so small as to be indivisible. He called these small pieces of matter 'atomos', the Greek word for indivisible. In addition, Democritus believed that the atoms differed in size and shape, were in constant motion in a void, collided with each other; and during these collisions, could rebound or stick together. Therefore, changes in matter were a result of dissociations or combinations of the atoms as they moved throughout the void. Here is some more background on his theory, Aristotle's idea and what happened next in atomic theory.

Interestingly, enough, Demokritos based his theory largely on his desire to prove another philosopher wrong, namely Zeno. Zeno theorized that motion is nothing but an illusion. Demokritos's atomic theory is based upon the notion that everything is made up of atoms that are eternally in motion, thus rendering Zeno's theory false. Zeno also stated that something can always be divided into smaller versions of the same thing, like the stone, while Demokritos stated that eventually there is nothingness in which atoms reside--this proving Zeno false again.

For a long time, we believed this was true. Now we know atoms are themselves made up of smaller particles, which require more and more energy for us to split into constituent components, in fact to so much more energy that the basement particle is the moment of the big bang, when it was a single point. This seems to prove Zeno right, in a very roundabout way: if the smallest base point is a single infinite point, there is no movement, only abstracted illusion.