ABSTRACT
This chapter introduces Bayes’ Theorem, named in honour of the Reverend Thomas Bayes. Bayes’ Theorem offers a way to update the probability of a hypothesis being true, given some new evidence, using a simple but very powerful mathematical equation. The Bayes Factor is thus the ratio of the likelihood of one hypothesis to the likelihood of the other. It is essentially a measure of which hypothesis better explains the world, given the evidence. Laplace's Rule of Succession, named after Pierre-Simon Laplace, is a rule-of-thumb way of calculating how likely it is that something that has happened before will happen again. Bayes' Theorem makes clear the importance of not just new evidence but also the (prior) probability that the hypothesis was true before the arrival of the new evidence. Bayes' Theorem allows the reader a way to update the probability that a hypothesis is true.
