ABSTRACT
In chapter 7.II an argument was given for the so-called pro rata rule, and hence for conditionalisation. The argument equates all conditional probabilities given any proposition B with those in a sample space B derived from a prior space K by eliminating all the points in K which are not in B. In the example used to illustrate this argument, K is the set {1,2,3,4,5,6} of possible results of throwing a die, and B is the subset, {1,3,5}, of the points in K which make true the proposition B that the result of the throw is an odd number.
