Bayes had an amazing insight that completely changed our reasoning about probabilities. This makes it easier to think about one’s chances in gambling and dealing in insurance. Recently, Bayes’ theorem was used to filter spam e-mail. This paper, published after Bayes’ death, was known by specialists, but now Google shows over 5 million hits if you search for ‘Bayesian’. But what is so groundbreaking about the Reverend Thomas Bayes’ concept? Without it our way of doing science would be much less accurate. Furthermore, understanding probabilities in the Bayesian way probably comes closest to the most fundamental thing that the brain has to do. The Bayesian approach is therefore at the heart of current models about the brain.