 The method for assigning probabilities Carnap used, was not universal and depended heavily on the code systems used. A general theory of induction using Bayes’ rule can only be developed when we can assign a universal probability to “any possible string” of symbols. In a paper in 1960 Solomonoff (1960, 1964a,b) was the first to sketch an outline of a solution for this problem. He formulated the notion of what is now called a universal probability distribution: consider the set of all possible finite strings to be programs for a universal Turing machine U and define the probability of a string x of symbols in terms of the length of the shortest program p that outputs x on U.