Aristotle was a pupil of Plato, but the two men differed significantly in their approach to the understanding of the natural world. Plato was an idealist who focused on mathematics as the underlying reality, whereas Aristotle took a more pragmatic approach and emphasized physical aspects (e.g. cause and effect); he searched for reasons why things were as they were. Aristotle's modification of Eudoxus' system (he described his cosmology in the work On the Heavens, or De caelo in Latin) was not undertaken from the point of view of a mathematical astronomer, but as part of his bold attempt to unify all the separate branches of natural philosophy. As mentioned above, it is unclear whether Eudoxus regarded the spheres in his system as mathematical constructions or material entities, but in Aristotle's version the spheres were regarded as real physical objects, motion being imparted to the planets by the divine and eternal 'ether' (a word derived from the Greek for 'runs always') which filled the spheres. This insistence on the physical reality of the spheres led to the addition of many more spheres to

The status of Eudoxus' homocentric spheres as a scientific theory is discussed and compared with later models in O'Neil (1969).

Callippus' model, twenty-two in all, so as to undo the motion of each planet before beginning the set of spheres associated with the next planet.

Aristotle wrote on many other subjects (e.g. physics, chemistry and biology) and attempted, with a considerable degree of success, to organize all knowledge into a unified whole. As a result, it became very difficult to criticize one aspect of Aristotle's teachings without criticizing it all. This led to a degree of stability and is one of the reasons why Aristotelian cosmology had such a pervasive influence. Central to Aristotle's teaching was the idea of a stationary Earth at the centre of the Universe:

... the natural movement, both of parts of the earth and of the earth as a whole, is toward the centre of the universe - this is why the earth now actually lies at the centre ... It happens that the same point is the centre both of the earth and of the universe. ... It is clear from these considerations that the earth neither moves nor

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