A Treatise on Probability

In this path breaking contribution to the logic of probability, Keynes showed how to adapt the work of George Boole for the purpose of estimating probabilities. Keynes is the first scholar in history to explicitly emphasize the importance of interval estimates in decision making. For Keynes there are only two types of probability estimates, point estimates and interval estimates. Unfortunately, Keynes decided to call interval estimates "non-numerical"probabilities. His reasoning is really quite obvious. A precise estimate of probability used a single numeral for the point estimate. Therefore, an imprecise estimate of probability used two numerals to denote an interval(set). Thus, an interval estimate is not based on a single numeral but two. These types of probabilities are thus "non-numerical"because you are not using a single numeral.