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On reflection scientists have rejected many past theories. It is also true that those theories were highly effective within the contexts that they had been developed and tested. The old earth centred model of the solar system was supremely accurate to predict locations of stars and planets. Even now Newton's law of gravity is used to calculate satellite trajectories and go to the moon. Beliefs are made on the basis of evidence on hand. Philosophers and statisticians have over a period time, developed thinking ways about the relationship between belief, action and evidence that captures this pragmatism. Beliefs come in degrees and measure how likely we think something to be true. The evidence we gather should influence these degrees of belief. The character of evidence makes us confident. We can use a branch of probability theory to map out a precise relationship between what we observe and what we ought to believe, A formula, known as Bayes' rule allows you to calculate your degree of belief or credence. This is after learning of some evidence that takes into account what you believed before you saw the evidence. Bayes' rule is a unique and rational way to update your beliefs.