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Aristotle's Explanation of Motion
Naturally, things tend to go where they belong, then stay at rest. Heavy
things go down (gravity); light things like smoke, air, and fire go up (levity).
All other motion is forced motion.
The ancient Greek philosophers posed the first recorded questions as to
the nature of the physical world. There was a firm belief in the ultimate
simplicity of the world - that in the end, it would be found that only a few
things were the basis of all the matter and phenomena that we see around
us. There arose two schools of thought: the idea of the 'atom', and the idea
of the 'elements'.
The most popular theory at the time was Aristotle's theory of the elements.
He believed that one could divide up a piece of matter an infinite number of
times, that is, one never came up with a piece of matter that could not be
further divided. He suggested that everything in the world be made up of
some combination of four elements: earth, fire, water, and air. The two
forces of gravity and levity acted upon the elements. Gravity was the
tendency for earth and water to sink and levity the tendency for air and fire
to rise.
A man named Democritus headed the second theory. He believed in the
existence of an 'elementary particle', which he called the 'atom'. These
atoms were indivisible, the smallest particle possible. He suggested that
there were many different types of unchangeable atoms, each with its own
shape and size, in constant motion. Matter, therefore, was made up of
large numbers of different types of atoms.
Because Aristotle's theory was highly popular, Democritus' theory of atoms
soon faded into obscurity, and was largely forgotten by the time of Christ.
The atomic concept of material as we know it today really only came into
being less than two hundred years ago.
Reference:
1. members.tripod.com/~Killashandra/Page2.html
2.
www.wooster.edu/Physics/Ludwigsen/lectures/111LEC16-1.html