More than 200 years ago, the Reverend Thomas Bayes described a general solution for the computation of conditional probabilities in an excellent article entitled "An essay towards solving a problem in the doctrine of changes".

This article was published after Bayes' death in 1763 in the Philosophical Transactions of the Royal Society, London (53, 370-518).

The method described by Bayes is of pivotal importance for the theory of evaluation of diagnostic tests. The term "theorem" is not really justified; in fact, Bayes' formula is a mathematical truism.