Theories of gravitation
|General relativity||History Topics Index|
Some existing things are natural, while others are due to other causes. Those that are natural are ... the simple bodies such as earth, fire, air and water; for we say that these things and things of this sort are natural. All these things evidently differ from those that are not naturally constituted, since each of them has within itself a principle of motion and stability in place ... A nature is a type of principle and cause of motion and stability within these things to which it primarily belongs ... A nature, then, is what we have said; and the things that have a nature are those that have this sort of principle. All things are substances, for a substance is a sort of subject, and a nature is invariably in a subject. The things that are in accordance with nature include both these and whatever belongs to them in their own right, as travelling upward belongs to fire ...So Aristotle argues that the stone falls because it has a "nature within it" which causes its motion to its natural place which is the centre of the Earth. Natural motion of the heavenly bodies, according to Aristotle, is circular. Other ideas which he put forward were that an object moving at constant speed requires a continuous force acting on it to maintain that speed. He also argued that force can only be applied to an object through contact.
As well as natural motion there is unnatural motion, as for example when a stone is thrown upwards. However such unnatural motions are short lived (since continual application of force is required to maintain motion) and the "natural desire" of the stone to return to the Earth takes over and then natural motion returns the stone to Earth. This theory survived through the 16th century and the following illustration from a book published in 1547 shows how Aristotle's theory was applied to cannonballs. In this illustration the cannonball first travels in a straight line due to its initial unnatural motion, then when this force is spent the cannonball follows the arc of a circle until it begins its natural motion vertically downwards to the Earth.