The definition of the term “probability” is almost as illusive as that of the term “electricity.” Much as the average person knows that the effect of an electric current is to produce certain phenomena with which he is familiar, so he understands in a general manner what is implied by the words “the probability that an event will take place.” The term is employed colloquially in many instances in which the quantitative idea is missing. “ Kain is probable today”; “He will probably be late”; and so on. Frequently, however, some measure of probability is implicit. For example, if the odds against a specified horse are quoted as being longer than the odds against another horse, it is understood that the chances in favour of the second horse are greater than those in favour of the first. It is to be supposed that it is more probable that the second horse will win than will the first, and a certain amount of arithmetical work has been necessary in order to obtain the figures for the odds.