• Bayes' theorem states that the best estimate of P(A|B) is the proportion of trials in which A occurred with B out of all the trials in which B occurred.• In plain language, this tells us that if we know event B occurred, the probability of event A is higher the more often that A and B occur together each time B is observed.• In a way, this adjusts P(A ∩ B) for the probability of B occurring; if B is extremely rare, P(B) and P(A ∩ B) will always be small; however, if A and B almost always happen together, P(A|B) will be high regardless of the probability of B.