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In order to talk about the nature of hte universe and to discuss question such as whether it has a begining or an end,you have to be clear about what s scientifc 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 a set of rules that relate quantities in the model to oservations that we make .It esists only in our minds and does not have any other reality.A theory is good theory if it satisfies two requirements :it must accurently 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 every thing was made out of four eliments earth,air,fire,water was simple enough to qualify,but it didnot make any defintire 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 quatity called their mass and inversely propotional to the square of the distance between them.yet it predicts the motions of the sun the moon and the planets to a high degree of accurance.