Bayes rule is a simple 17th century theory for the evaluation of knowledge, criticized for most of the 20th century. It helps people evaluate their initial ideas, update and modify them with new information, in order to make better decisions. Bayesian search theory was developed by two clergymen and amateur mathematicians, the Englishman Thomas Bayes and his Welsh friend Richard Price, during the 18th century. Today we use it for an immense amount of things, such as filtering spam and training autopilot systems.