The law of total probability is[1] a theorem that states, in its discrete case, if { B n : n = 1 , 2 , 3 , … } {\displaystyle \left\{{B_{n}:n=1,2,3,\ldots }\right\}} is a finite or countably infinite partition of a sample space (in other words, a set of pairwise disjoint events whose union is the entire sample space) and each event B n {\displaystyle B_{n}} is measurable, then for any event A {\displaystyle A} of the same probability space: