Thomas Bayes (1702-1761)
Thomas Bayes was a English minister and a mathmatician. In his Essay Towards Solving a Problem in the Doctrine of Chances (published after his death) he defined a method for inferring the probability of the occurance of an event based on the frequency of past occurance of that event. This work laid the foundation of modern Bayesian statistics and reasoning under certainty. For indepth biographical information on Bayes go to--
http://turnbull.dcs.st-and.ac.uk/~history/Mathematicians/Bayes.html or