In books on the calculus of probability, there have been many accounts as to what is the meaning of the term “probable.” We can readily divide them into three groups. The first sometimes defines probability in terms of the ratio between the number of cases favorable to an event and the number of equally possible cases. Sometimes probability is defined in some way other than this, but the above formulation, or one similar to it is used to describe the “measure of probability.” This concept is what is called “the classical concept” of probability and was held by the great workers in the field of probability from the beginning of the eighteenth and opening of the nineteenth centuries, including James Bernoulli (1713), Thomas Bayes (1763), and the Marquis de Laplace (1812). The second conceives of probability as a logical relation between hypothesis and evidence. Following this conception, one could identify “probability” with “degree of confirmation.” We will term this the “logical concept.” The third group identified probability with the relative frequency with which a property occurs in a specified class of elements. We will call this the “frequency concept.”