Aristotle's Physics was written in the fourth century BC. For more than two thousand years this book served as the basis of natural philosophy (physics), throughout the Middle Ages and the early modern period, up to the sixteenth century the time of Galileo.

In this book, Aristotle explains the main principles of his natural philosophy. The book deals with the subject of motion and its causes, the motion of objects from one place to another, and motion as a general concept of change of state. According to Aristotle, the motion of physical bodies is of two types: natural motion and violent motion.

Natural motion is the motion arising from the nature of an object. This motion does not require an external cause in order to occur. For example: Heavy bodies naturally move toward the center of the earth, therefore falling is a natural motion. The natural motion of objects is one of the characteristics of an object. For example: The natural movement of the celestial bodies made of ether is circular (see the Ptolemaic planetary system) rather than a movement toward the center of the earth.

Violent motion, by contrast, is motion contrary to the nature of the object and requires an external force in order to occur. Most motions are of this type. A stone thrown into the air moves in a violent motion, against its nature and therefore requires force in order to cause it to move. In any discussion of violent motion, one must find the reason (the force) which causes this motion, as long as it exists.

Although Galileo rejected Aristotle's theory, he learned a great deal from the books of Aristotle and his school, and even preserved some of the Aristotelian concepts, most of which were subsequently abandoned by modern science.

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