Probability, Axioms, Bayes Theorem, Frequentists, Bayesian.
Kenny is a shark (A), i.e. we want to calculate the probability Kenny is a shark when we know he has big, sharp teeth (A | B). By construction (due to the 3rd axiom), the multiplication rule is symmetric in A and B, so P(A & B) = P(A | B) P(B) = P(B | A) P(A) Rearranging the rightmost equality