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Various translations of some of Aristotle's works   


Aristotle's book The Physics, was in existence by about 350 B.C. This book is mainly concerned with change and how change exists in all of its various forms, such as local motion and the different types of changes involved in processes such as burning, growing, cooking, etc. In The Physics Aristotle deduces that there must be two main principles of change. These principles are 1) matter and 2) form. Matter stays the same in general composition throughout change much as a rock would stay the same even after being broken into several small pieces. Form, however, is what changes through various processes.
In order to prevent an infinite regression of argument, Aristotle came up with the his famous idea of the "Prime Mover," which exists outside the earth somewhere in the heavens and is ultimately responsible for all change on earth.
Aristotle's physics separated the universe into two main areas: the terrestrial realm and the celestial realm. The terrestrial realm was composed of the four elements: earth, wind, water and fire, which could each be either hot, wet, dry or cold, and the celestial realm made of ether, or what Aristotle called the "quintessence."
                                                                                                                              
                                     
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Diagram of Aristotle's proposed classification of the 4 elements


The basic assumption of Aristotelian physics was that the natural state of terrestrial matter is at rest, and that earth air and water would continually strive to reach their natural place at the center of the earth unless impeded by an impenetrable surface like the ground or a ceiling. He assumed that the natural resting place of fire was somewhere above the earth but below the moon. this model held that the complicated nature of the circulation of the air was a result of the conflict between fire, which was trying to rise, and air which was trying to fall.
Aristotle's model provided simple explanations for natural movement such as the falling of rocks, the falling of water, the rising of flames, and the circulation of air, but it was less accurate in describing different types of motion.
For example, this model could not successfully describe why an arrow shot from a bow would continue to move after it had been released from its driving force (Aristotle claimed that objects could only stay in motion while an outside force was acting on them). If the natural state of rest of the arrow is near the earth, then why wouldn't the arrow fall immediately to the ground after being released from the bow. For these situations Aristotelian physics had to describe motion as "violent" because it was different in that the movement was not initially directed toward the center of the earth. Elaborate theories were proposed to explain these types of movement. It was postulated that the air in front of the moving object was disturbed by the unnatural movement and it would swirl behind the object and create a force to push it along.

The physics of dynamics as proposed by Aristotle assujmed the the earth and the heavens were subjected to two different sets of laws. For terrestrial dynamics there were both natural and forced motion. Natural motions consisted of straight line, vertical and towards natural place motions, and forced motions were those that were discontinuous, and not towards the natural resting place. Celestial dynamics were considered always natural, circular and continuous. The movements of the celestial bodies were thought to move in this manner because the Prime Mover provided the constant and unchanging force to their paths of motion.

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