 Von Mises never totally formalized his definition of a proper selection rule for sub-sequences, but in 1940 Alonzo Church defined it as any recursive function which having read the first N elements of the sequence decides if it wants to select element number "N" + 1.Church was a pioneer in the field of computable functions, and the definition he made relied on the Church Turing Thesis for computability.This definition is often called "Mises–Church randomness".