求翻译：in ｏrder to talk about the nature of the universe ａnd to discuss questions such as whether it has a beginning ｏr an end, you have to be clear about what a scientific theory is. i shall take the simple-minded view that a theory is just a model of the universe, ｏr a restricted part of it, ａnd a set of rules that relate quantities in the model to observations that we make. it exists only in our minds ａnd does not have any other reality. a theory is a good theory if it satisfies two requirements. it must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, ａnd it must make definite predictions about the results of future observations. for example, aristotle’s theory that everything was made out of four elements, earth, air, fire, ａnd water, was simple enough to qualify, but it did not make any definite predictions. on the other hand, newton’s theory of gravity was based on an even simpler model, in which bodies attracted each other with a force that was proportional to a quantity called their mass ａnd inversely proportional to the square of the distance between them. yet it predicts the motion of the sun, the moon, ａnd the planets to a high degree of accuracy.是什么意思？

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