In which I say a few words on Bayes' Theorem

From the definition of Bayes' Theorem in Wikipedia, 

In probability theory and applications, Bayes' theorem shows the relation between two conditional probabilities which are the reverse of each other.

 Let's go back and look at what a conditional probability is. 

Conditional probability is the probability of some event A, given the occurrence of some other event B. 

Probability of event A given the occurrence of B is written as P(A|B).